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1.	Ajitha, D, et al. (författare) Spin-orbit ab initio study of alkyl halide dissociation via electronic curve crossing 2004 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 121:12, s. 5761-5766 Tidskriftsartikel (refereegranskat)abstract An ab initio study of the role of electronic curve crossing in the photodissociation dynamics of the alkyl halides is presented. Recent experimental studies show that curve crossing plays a deterministic role in deciding the channel of dissociation. Coupled repulsive potential energy curves of the low-lying n-sigma* states are studied including spin-orbit and relativistic effects. Basis set including effect of core correlation is used. Ab initio vertical excitation spectra of CH3I and CF3I are in agreement with the experimental observation. The curve crossing region is around 2.371 Angstrom for CH3I and CF3I. The potential curves of the repulsive excited states have larger slope for CF3I, suggesting a higher velocity and decreased intersystem crossing probability on fluorination. We also report the potential curves and the region of curve crossing for CH3Br and CH3Cl.
2.	Al-Abdalla, A, et al. (författare) Ab initio model potential embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6 1998 Ingår i: Journal of Chemical Physics. - : Springer Science and Business Media LLC. - 0021-9606 .- 1089-7690. ; 108, s. 2005-2014 Tidskriftsartikel (refereegranskat)abstract In this paper we present the results of an ab initio model potential (AIMP) embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6; complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on CrCl63- and CrBr63- clusters embedded in ab initio model potential representations of the surrounding lattices Cs2NaYCl6 and Cs2NaYBr6. The experimental structural data are revisited and some new results are found which differ significantly from those available in the literature. The calculated local structure parameters and electronic transition energies which can be compared to experiments are found to be very good; new structural and spectroscopic results are produced which have been neither measured nor calculated, which are complementary to the available ones, and whose quality is expected to be high as well. In particular, the question of the competition of the excited-state absorptions with the potential vibronic laser emission has been adressed: A considerable overlap between the broad E-2(g) –> (2)A(1g) excited-state absorption and (4)A(2g) <– T-4(2g) emission bands is predicted in both materials, which must result in a reduction in the emission efficiency. Finally, it is shown that the quantum mechanical embedding effects due to the fact that the external Cs+, Na+, Y3+, Cl-, and Br- ions are not point charges, are non-negligible; lacking of these effects must be one of the reasons which make previous Density Functional Theory calculations show significantly larger discrepancies with the available experiments.
3.	Aquilante, Francesco, et al. (författare) Cholesky decomposition techniques in electronic structure theory 2011 Ingår i: Linear-Scaling Techniques in Computational Chemistry and Physics. - Dordrecht : Springer. - 9789048128525 ; , s. 301-304 Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract We review recently developed methods to efficiently utilize the Cholesky decomposition technique in electronic structure calculations. The review starts with a brief introduction to the basics of the Cholesky decomposition technique. Subsequently, examples of applications of the technique to ab inito procedures are presented. The technique is demonstrated to be a special type of a resolution-of-identity or density-fitting scheme. This is followed by explicit examples of the Cholesky techniques used in orbital localization, computation of the exchange contributionto the Fock matrix, in MP2, gradient calculations, and so-called method specific Cholesky decomposition. Subsequently, examples of calibration of the method with respect to computed total energies, excitation energies, and auxiliary basis set pruning are presented. In particular, it is demonstrated that the Cholesky method is an unbiased method to derive auxiliary basis sets. Furthermore, details of the implementational considerations are put forward and examples from a parallel Cholesky decomposition scheme is presented. Finally, an outlook and perspectives are presented, followed by a summary and conclusions section. We are of the opinion that the Cholesky decomposition method is a technique that has been overlooked for too long. We have just recently started to understand how to efficiently incorporate the method in existing ab initio programs. The full potential of the Cholesky technique has not yet been fully explored.
4.	Aquilante, Francesco, et al. (författare) Inner projection techniques for the low-cost handling of two-electron integrals in quantum chemistry 2017 Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 115:17-18, s. 2052-2064 Tidskriftsartikel (refereegranskat)abstract The density-fitting technique for approximating electron-repulsion integrals relies on the quality of auxiliary basis sets. These are commonly obtained through data fitting, an approach that presents some shortcomings. On the other hand, it is possible to derive auxiliary basis sets by removing elements from the product space of both contracted and primitive orbitals by means of a particular form of inner projection technique that has come to be known as Cholesky decomposition (CD). This procedure allows for on-the-fly construction of auxiliary basis sets that may be used in conjunction with any quantum chemical method, i.e. unbiased auxiliary basis sets. One key feature of these sets is that they represent the electron-repulsion integral matrix in atomic orbital basis with an accuracy that can be systematically improved. Another key feature is represented by the fact that locality of fitting coefficients is obtained even with the long-ranged Coulomb metric, as result of integral accuracy. Here we report on recent advances in the development of the CD-based density fitting technology. In particular, the implementation of analytical gradients algorithms is reviewed and the present status of local formulations - potentially linear scaling - is analysed in detail.
5.	Aquilante, Francesco, et al. (författare) Low-cost evaluation of the exchange Fock matrix from Cholesky and density fitting representations of the electron repulsion integrals 2007 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 126:19, s. 194106- Tidskriftsartikel (refereegranskat)abstract The authors propose a new algorithm, “local K” (LK), for fast evaluation of the exchange Fock matrix in case the Cholesky decomposition of the electron repulsion integrals is used. The novelty lies in the fact that rigorous upper bounds to the contribution from each occupied orbital to the exchange Fock matrix are employed. By formulating these inequalities in terms of localized orbitals, the scaling of computing the exchange Fock matrix is reduced from quartic to quadratic with only negligible prescreening overhead and strict error control. Compared to the unscreened Cholesky algorithm, the computational saving is substantial for systems of medium and large sizes. By virtue of its general formulation, the LK algorithm can be used also within the class of methods that employ auxiliary basis set expansions for representing the electron repulsion integrals.
6.	Aquilante, Francesco, et al. (författare) Modern quantum chemistry with [Open]Molcas 2020 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:21 Tidskriftsartikel (refereegranskat)abstract MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.
7.	Aquilante, Francesco, et al. (författare) Software news and update MOLCAS 7 : The Next Generation 2010 Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 31:1, s. 224-247 Forskningsöversikt (refereegranskat)abstract Some of the new unique features of the MOLCAS quantum chemistry package version 7 are presented in this report. In particular, the Cholesky decomposition method applied to some quantum chemical methods is described. This approach is used both in the context of a straight forward approximation of the two-electron integrals and in the generation of so-called auxiliary basis sets. The article describes how the method is implemented for most known wave functions models: self-consistent field, density functional theory, 2nd order perturbation theory, complete-active space self-consistent field multiconfigurational reference 2nd order perturbation theory, and coupled-cluster methods. The report further elaborates on the implementation of a restricted-active space self-consistent field reference function in conjunction with 2nd order perturbation theory. The average atomic natural orbital basis for relativistic calculations, covering the whole periodic table, are described and associated unique properties are demonstrated. Furthermore, the use of the arbitrary order Douglas-Kroll-Hess transformation for one-component relativistic calculations and its implementation are discussed. This section especially focuses on the implementation of the so-called picture-change-free atomic orbital property integrals. Moreover, the ElectroStatic Potential Fitted scheme, a version of a quantum mechanics/molecular mechanics hybrid method implemented in MOLCAS, is described and discussed. Finally, the report discusses the use of the MOLCAS package for advanced studies of photo chemical phenomena and the usefulness of the algorithms for constrained geometry optimization in MOLCAS in association with such studies.
8.	Aspuru-Guzik, Alan, et al. (författare) The Matter Simulation (R)evolution 2018 Ingår i: ACS CENTRAL SCIENCE. - : American Chemical Society (ACS). - 2374-7943 .- 2374-7951. ; 4:2, s. 144-152 Tidskriftsartikel (refereegranskat)abstract To date, the program for the development of methods and models for atomistic and continuum simulation directed toward chemicals and materials has reached an incredible degree of sophistication and maturity. Currently, one can witness an increasingly rapid emergence of advances in computing, artificial intelligence, and robotics. This drives us to consider the future of computer simulation of matter from the molecular to the human length and time scales in a radical way that deliberately dares to go beyond the foreseeable next steps in any given discipline. This perspective article presents a view on this future development that we believe is likely to become a reality during our lifetime.
9.	Augusto, Felipe A., et al. (författare) Mechanism of activated chemiluminescence of cyclic peroxides : 1,2-dioxetanes and 1,2-dioxetanones 2017 Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 19:5, s. 3955-3962 Tidskriftsartikel (refereegranskat)abstract Almost all chemiluminescent and bioluminescent reactions involve cyclic peroxides. The structure of the peroxide and reaction conditions determine the quantum efficiency of light emission. Oxidizable fluorophores, the so-called activators, react with 1,2-dioxetanones promoting the former to their first singlet excited state. This transformation is inefficient and does not occur with 1,2-dioxetanes; however, they have been used as models for the efficient firefly bioluminescence. In this work, we use the SA-CASSCF/CASPT2 method to investigate the activated chemiexcitation of the parent 1,2-dioxetane and 1,2-dioxetanone. Our findings suggest that ground state decomposition of the peroxide competes efficiently with the chemiexcitation pathway, in agreement with the available experimental data. The formation of non-emissive triplet excited species is proposed to explain the low emission efficiency of the activated decomposition of 1,2-dioxetanone. Chemiexcitation is rationalized considering a peroxide/activator supermolecule undergoing an electron-transfer reaction followed by internal conversion.
10.	Aurbakken, Einar, et al. (författare) Transient spectroscopy from time-dependent electronic-structure theory without multipole expansions 2024 Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : American Physical Society. - 2469-9926 .- 2469-9934. ; 109:1 Tidskriftsartikel (refereegranskat)abstract Based on the work done by an electromagnetic field on an atomic or molecular electronic system, a general gauge-invariant formulation of transient absorption spectroscopy is presented within the semiclassical approximation. Avoiding multipole expansions, a computationally viable expression for the spectral response function is derived from the minimal-coupling Hamiltonian of an electronic system interacting with one or more laser pulses described by a source-free, enveloped electromagnetic vector potential. With a fixed-basis expansion of the electronic wave function, the computational cost of simulations of laser-driven electron dynamics beyond the dipole approximation is the same as simulations adopting the dipole approximation. We illustrate the theory by time-dependent configuration interaction and coupled-cluster simulations of core-level absorption and circular dichroism spectra.
11.	Bao, Jie J., et al. (författare) Analytic gradients for compressed multistate pair-density functional theory 2022 Ingår i: Molecular Physics. - : Taylor & Francis Group. - 0026-8976 .- 1362-3028. ; 120:19-20 Tidskriftsartikel (refereegranskat)abstract Photochemical reactions often involve states that are closely coupled due to near degeneracies, for example by proximity to conical intersections. Therefore, a multistate method is used to accurately describe these states; for example, ordinary perturbation theory is replaced by quasidegenerate perturbation theory. Multiconfiguration pair-density functional theory (MC-PDFT) provides an efficient way to approximate the full dynamical correlation energy of strongly correlated systems, and we recently proposed compressed multistate pair-density functional theory (CMS-PDFT) to treat closely coupled states. In the present paper, we report the implementation of analytic gradients for CMS-PDFT in both OpenMolcas and PySCF, and we illustrate the use of these gradients by applying the method to the excited states of formaldehyde and phenol.
12.	Barnes, Leslie A, et al. (författare) Bond length, dipole moment, and harmonic frequency of CO 1993 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:5, s. 3972-3977 Tidskriftsartikel (refereegranskat)abstract A detailed comparison of some properties of CO is given, at the modified coupled-pair functional, single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), using a variety of basis sets. With very large one-particle basis sets, the CCSD(T) method gives excellent results for the bond distance, dipole moment, and harmonic frequency of CO. In a [6s 5p 4d 3f 2g 1h] + (1s 1p 1d) basis set, the bond distance is about 0.005a0 too large, the dipole moment about 0.005 a.u. too small, and the frequency about 6 cm-1 too small, when compared with experimental results.
13.	Barnes, Leslie A, et al. (författare) Structure and energetics of Cr(CO)6 and Cr(CO)5 1993 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:5, s. 3978-3989 Tidskriftsartikel (refereegranskat)abstract The geometric structure of Cr(CO)6 is optimized at the modified coupled-pair functional (MCPF), single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), and the force constants for the totally symmetric representation are determined. The geometry of Cr(CO)5 is partially optimized at the MCPF, CCSD, and CCSD(T) levels of theory. Comparison with experimental data shows that the CCSD(T) method gives the best results for the structures and force constants, and that remaining errors are probably due to deficiencies in the one-particle basis sets used for CO. The total binding energies of Cr(CO)6 and Cr(CO)5 are also determined at the MCPF, CCSD, and CCSD(T) levels of theory. The CCSD(T) method gives a much larger total binding energy than either the MCPF or CCSD methods. An analysis of the basis set superposition error (BSSE) at the MCPF level of treatment points out limitations in the one-particle basis used here and in a previous study. Calculations using larger basis sets reduce the BSSE, but the total binding energy of Cr(CO)6 is Still Significantly smaller than the experimental value, although the first CO bond dissociation energy of Cr(CO)6 is well described. An investigation of 3s3p correlation reveals only a small effect. In the largest basis set, the total CO binding energy of Cr(CO)6 is estimated to be 140 kcal/mol at the CCSD(T) level of theory, or about 86% of the experimental value. The remaining discrepancy between the experimental and theoretical value is probably due to limitations in the one-particle basis, rather than limitations in the correlation treatment. In particular, an additional d function and an f function on each C and 0 are needed to obtain quantitative results. This is underscored by the fact that even using a very large primitive set (1042 primitive functions contracted to 300 basis functions), the superposition error for the total binding energy of Cr(CO)6 is 22 kcal/mol at the MCPF level of treatment.
14.	Barnes, Leslie A, et al. (författare) Symmetry breaking in O+4 : an application of the Brueckner coupled-cluster method 1994 Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 223:3, s. 207-214 Tidskriftsartikel (refereegranskat)abstract A recent calculation of the antisymmetric stretch frequency for the rectangular structure of quartet O4+ using the QCISD(T) method gave a value of 3710 cin-1. This anomalous frequency is shown to be a consequence of symmetry-breaking effects, which occur even though the QCISD(T) solution derived from a delocalized SCF reference function lies energetically well below the two localized (symmetry-broken) solutions at the equilibrium geometry. The symmetry breaking is almost eliminated at the CCSD level of theory, but the small remaining symmetry-breaking effects are magnified at the CCSD (T) level of theory so that the antisymmetric stretch frequency is still significantly in error. The Brueckner coupled-cluster method, however, leads to a symmetrical solution which is free of symmetry-breaking effects, with an antisymmetric stretch frequency of 1322 cm-1, in good agreement with our earlier calculations using the CASSCF/CASSI method.
15.	Battaglia, Stefano, et al. (författare) Extended Dynamically Weighted CASPT2 : The Best of Two Worlds 2020 Ingår i: Journal of Chemical Theory and Computation. - : AMER CHEMICAL SOC. - 1549-9618 .- 1549-9626. ; 16:3, s. 1555-1567 Tidskriftsartikel (refereegranskat)abstract We introduce a new variant of the complete active A space second-order perturbation theory (CASPT2) method that performs similarly to multistate CASPT2 (MS-CASPT2) in regions of the potential energy surface where the electronic states are energetically well separated and is akin to extended MS-CASPT2 (XMS-CASPT2) in case the underlying zeroth-order references are near-degenerate. Our approach follows a recipe analogous to that of XMS-CASPT2 to ensure approximate invariance under unitary transformations of the model states and a dynamic weighting scheme to smoothly interpolate the Fock operator between state-specific and state-average regimes. The resulting extended dynamically weighted CASPT2 (XDW-CASPT2) methodology possesses the most desirable features of both MS-CASPT2 and XMS-CASPT2, that is, the ability to provide accurate transition energies and correctly describe avoided crossings and conical intersections. The reliability of XDW-CASPT2 is assessed on a number of molecular systems. First, we consider the dissociation of lithium fluoride, highlighting the distinctive characteristics of the new approach. Second, the invariance of the theory is investigated by studying the conical intersection of the distorted allene molecule. Finally, the relative accuracy in the calculation of vertical excitation energies is benchmarked on a set of 26 organic compounds. We found that XDW-CASPT2, albeit being only approximately invariant, produces smooth potential energy surfaces around conical intersections and avoided crossings, performing equally well to the strictly invariant XMS-CASPT2 method. The accuracy of vertical transition energies is almost identical to MS-CASPT2, with a mean absolute deviation of 0.01-0.02 eV, in contrast to 0.12 eV for XMS-CASPT2.
16.	Battaglia, Stefano, et al. (författare) On the role of symmetry in XDW-CASPT2 2021 Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 154:3 Tidskriftsartikel (refereegranskat)abstract Herewith, we propose two new exponents for the recently introduced XDW-CASPT2 method [S. Battaglia and R. Lindh, J. Chem. Theory Comput. 16, 1555-1567 (2020)], which fix one of the largest issues hindering this approach. By using the first-order effective Hamiltonian coupling elements, the weighting scheme implicitly takes into account the symmetry of the states, thereby averaging Fock operators only if the zeroth-order wave functions interact with each other. The use of Hamiltonian couplings also provides a physically sounder approach to quantitate the relative weights; however, it introduces new difficulties when these rapidly die off to zero. The improved XDW-CASPT2 method is critically tested on several systems of photochemical relevance, and it is shown that it succeeds in its original intent of maintaining MS-CASPT2 accuracy for the evaluation of transition energies and at the same time providing smooth potential energy surfaces around near-degenerate points akin to XMS-CASPT2.
17.	Battaglia, Stefano, et al. (författare) Regularized CASPT2 : an Intruder-State-Free Approach 2022 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 18:8, s. 4814-4825 Tidskriftsartikel (refereegranskat)abstract In this work we present a new approach to fix the intruder-state problem (ISP) in CASPT2 based on sigma p regularization. The resulting sigma(p)-CASPT2 method is compared to previous techniques, namely, the real and imaginary level shifts, on a theoretical basis and by performing a series of systematic calculations. The analysis is focused on two aspects, the effectiveness of sigma(p)-CASPT2 in removing the ISP and the sensitivity of the approach with respect to the input parameter. We found that sigma p- CASPT2 compares favorably with respect to previous approaches and that different versions, sigma(1)-CASPT2 and sigma(2)-CASPT2, have different potential application domains. This analysis also reveals the unsuitability of the real level shift technique as a general way to avoid the intruder-state problem.
18.	Bernhardsson, Anders, et al. (författare) A direct implementation of the second-order derivatives of multiconfigurational SCF energies and an analysis of the preconditioning in the associated response equation 1999 Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 96:4, s. 617-628 Tidskriftsartikel (refereegranskat)abstract A direct implementation of the reduced multiplication scheme of the Rys-Gauss quadrature in the computation of the second-order geometric derivatives for MCSCF wavefunctions is presented. Characteristics unique to this implementation are simultaneous evaluation of first- and second-order two-electron integral derivatives, compact representation of intermediate elements and efficient prescreening. Finally, a novel technique for the preconditioning of the response equations is reported. Applications sizing up to 510 basis functions are included in the presentation.
19.	Bernhardsson, Anders, et al. (författare) Direct self-consistent reaction field with Pauli repulsion : Solvation effects on methylene peroxide 1996 Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 251:3-4, s. 141-149 Tidskriftsartikel (refereegranskat)abstract The properties of methylene peroxide in the gas phase and water have been examined. The solution effects have been modeled with a cavity. To make it possible to optimize the geometry of the cavity, we have added an approximate description of the Pauli repulsion between the solvent and the solute. An efficient direct implementation of the calculation of multicenter multipole operators is also described. We have applied the method to calculate the electric properties of methylene peroxide, a system that is characterized by its near degeneracy between a biradical and a zwitterionic state. As expected, it is shown that solvent effects will stabilize the ionic configuration, resulting in a considerably weakened OO bond and an increased double bond character in the CO bond. The theoretical study has been performed using the multiconfigurational (CAS) SCF method and second-order perturbation theory (CASPT2).
20.	Berraud-Pache, Romain, et al. (författare) QM/MM Study of the Formation of the Dioxetanone Ring in Fireflies through a Superoxide Ion 2018 Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 122:20, s. 5173-5182 Tidskriftsartikel (refereegranskat)abstract The bioluminescence emission from fireflies is an astounding tool to mark and view cells. However, the bioluminescent mechanism is not completely deciphered, limiting the comprehension of key processes. We use a theoretical approach to study for the first time the arrival of a dioxygen molecule inside the fireflies protein and one path of the formation of the dioxetanone ring, the high-energy intermediate precursor of the bioluminescence. To describe this reaction step, a joint approach combining classical molecular dynamics (MD) simulations and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations is used. The formation of the dioxetanone ring has been studied for both singlet and triplet states with the help of MS-CASPT2 calculations. We also emphasize the role played by the proteinic environment in the formation of the dioxetanone ring. The results obtained shed some light on an important reaction step and give new insights concerning the bioluminescence in fireflies.
21.	Boström, Jonas, et al. (författare) Ab Initio Density Fitting : Accuracy Assessment of Auxiliary Basis Sets from Cholesky Decompositions 2009 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 5:6, s. 1545-1553 Tidskriftsartikel (refereegranskat)abstract The accuracy of auxiliary basis sets derived by Cholesky decompositions of the electron repulsion integrals is assessed in a series of benchmarks on total ground state energies and dipole moments of a large test set of molecules. The test set includes molecules composed of atoms from the first three rows of the periodic table as well as transition metals. The accuracy of the auxiliary basis sets are tested for the 6-31 G**, correlation consistent, and atomic natural orbital basis sets at the Hartree-Fock, density functional theory, and second-order Moller-Plesset levels of theory. By decreasing the decomposition threshold, a hierarchy of auxiliary basis sets is obtained with accuracies ranging from that of standard auxiliary basis sets to that of conventional integral treatments.
22.	Boström, Jonas, et al. (författare) Calibration of Cholesky Auxiliary Basis Sets for Multiconfigurational Perturbation Theory Calculations of Excitation Energies 2010 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 6:3, s. 747-754 Tidskriftsartikel (refereegranskat)abstract The accuracy of auxiliary basis sets derived from Cholesky decomposition of two-electron integrals is assessed for excitation energies calculated at the state-average complete active space self-consistent field (CASSCF) and multiconfigurational second order perturbation theory (CASPT2) levels of theory using segmented as well as generally contracted atomic orbital basis sets. Based on 196 valence excitations in 26 organic molecules and 72 Rydberg excitations in 3 organic molecules, the results show that Cholesky auxiliary basis sets can be used without compromising the accuracy of the multiconfigurational methods. Specifically, with a decomposition threshold of 10(-4) au, the mean error due to the Cholesky auxiliary basis set is 0.001 eV, or smaller, decreasing with increasing atomic orbital basis set quality.
23.	Calio, Paul B., et al. (författare) Minimum-Energy Conical Intersections by Compressed Multistate Pair-Density Functional Theory 2024 Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 128:9, s. 1698-1706 Tidskriftsartikel (refereegranskat)abstract Compressed multistate pair-density functional theory (CMS-PDFT) is a multistate version of multiconfiguration pair-density functional theory that can capture the correct topology of coupled potential energy surfaces (PESs) around conical intersections. In this work, we develop interstate coupling vectors (ISCs) for CMS-PDFT in the OpenMolcas and PySCF/mrh electronic structure packages. Yet, the main focus of this work is using ISCs to calculate minimum-energy conical intersections (MECIs) by CMS-PDFT. This is performed using the projected constrained optimization method in OpenMolcas, which uses ISCs to restrain the iterations to the conical intersection seam. We optimize the S1/S0 MECIs for ethylene, butadiene, and benzene and show that CMS-PDFT gives smooth PESs in the vicinities of the MECIs. Furthermore, the CMS-PDFT MECIs are in good agreement with the MECI calculated by the more expensive XMS-CASPT2 method.
24.	Chen, Shu-Feng, et al. (författare) Multireference calculations on the chemical origin and mechanism of firefly bioluminescence 2010 Ingår i: Abstracts of the 16th International Symposium on Bioluminescence and Chemiluminescence – (ISBC 2010). - : Wiley. ; , s. 89-90 Konferensbidrag (övrigt vetenskapligt/konstnärligt)
25.	Chen, Shu-Feng, et al. (författare) Multireference theoretical studies on the solvent effect of firefly multicolor bioluminescence 2011 Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:13, s. 3371-3377 Tidskriftsartikel (refereegranskat)abstract In concert with the recent spectroscopic studies of the light-color modulation mechanism of firefly (Hirano et al., J Am Chem Soc 2009, 131, 2385), quantum chemical calculations using complete active space SCF (CASSCF), multistate complete active space second order perturbation (MS-CASPT2) theory as well as a time-dependent density functional theory (TD-DFT) approach with the Coulomb attenuated hybrid exchange-correlation functional (CAM-B3LYP) were performed on the excited state (S1) of the keto-form oxyluciferin (keto-OxyLH2). Benzene, DMSO, CH3CN, and H2O were chosen as polar solvents. The polarization effect of less polar solvent was considered by a simple model, complex of keto-OxyLH2, and NH3 with different covalent character. The calculated results supported the experimental conclusion: (1) the light emitter of bioluminescence is the S1 state of keto-OxyLH2 anion [(keto-1)], and (2) light emission from (keto-1) is modulated by the polarity of surrounding environment and the degree of covalent character of hydrogen bond between (keto-1)* and a protonated basic moiety. The mechanism of the multicolor bioluminescence was discussed from the theoretical viewpoint.
26.	Chen, Shu-Feng, et al. (författare) Systematic theoretical investigation on the light emitter of firefly 2011 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 7:3, s. 798-803 Tidskriftsartikel (refereegranskat)abstract This is a systematic theoretical investigation on all the possible light emitters of firefly using multireference method. Six chemical forms of oxyluciferin (OxyLH2) molecules/anions were studied by a multi-state complete active space second order perturbation (MS-CASPT2) method in vacuum and DMSO. The calculated results and subsequent analysis excluded enol-OxyLH2, keto-OxyLH2 and enolate-OxyLH- as possible light emitters. The remaining three candidates, phenolate-enol-OxyLH-, phenolate-keto-OxyLH- and OxyL2-, were further investigated in protein by a MS-CASPT2/molecular mechanics (MM) study to explain the natural bioluminescence of firefly. By comparison of the MS-CASPT2/MM calculated results of phenolate-enol-OxyLH-, phenolate-keto-OxyLH- and OxyL2- with the experimental observation and detailed analysis, we concluded that the direct decomposition excited-state product of firefly dioxetanone in vivo and the only light emitter of firefly in natural bioluminescence is the first singlet exited state (S1) of phenolate-keto-OxyLH-.
27.	Choe, Yong-Kee, et al. (författare) Theoretical study of the electronic ground state of iron(II) porphine. II 1999 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 111:9, s. 3837-3845 Tidskriftsartikel (refereegranskat)abstract Ten low-lying electronic states of Fe(II) porphine, (5)A(1g), E-5(g), B-5(2g), (3)A(2g), B-3(2g), E-3(g)(A), E-3(g)(B), (1)A(1g), B-1(2g), and E-1(g) states, are studied with multiconfigurational second-order perturbation (CASPT2) calculations with complete active space self-consistent field (CASSCF) reference functions with larger active space and basis sets. The enlargement of active space and basis sets has no influence on the conclusion of a previous multireference Moller-Plesset perturbation (MRMP) study. The present CASPT2 calculation concludes that the (5)A(1g) state is the ground state. A relativistic correction has been performed by the relativistic scheme of eliminating small components (RESC). For energetics, no significant contribution from the relativistic correction was found. The relative energies and orbital energies are not changed appreciably by the introduction of a relativistic correction. The present result does not agree with all the spectroscopic observations, but is consistent with a magnetic moment study.
28.	Cuéllar-Zuquin, Juliana, et al. (författare) Characterizing Conical Intersections in DNA/RNA Nucleobases with Multiconfigurational Wave Functions of Varying Active Space Size 2023 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 19:22, s. 8258-8272 Tidskriftsartikel (refereegranskat)abstract We characterize the photochemically relevant conical intersections between the lowest-lying accessible electronic excited states of the different DNA/RNA nucleobases using Cholesky decomposition-based complete active space self-consistent field (CASSCF) algorithms. We benchmark two different basis set contractions and several active spaces for each nucleobase and conical intersection type, measuring for the first time how active space size affects conical intersection topographies in these systems and the potential implications these may have toward their description of photoinduced phenomena. Our results show that conical intersection topographies are highly sensitive to the electron correlation included in the model: by changing the amount (and type) of correlated orbitals, conical intersection topographies vastly change, and the changes observed do not follow any converging pattern toward the topographies obtained with the largest and most correlated active spaces. Comparison across systems shows analogous topographies for almost all intersections mediating population transfer to the dark 1nO/Nπ* states, while no similarities are observed for the "ethylene-like" conical intersection ascribed to mediate the ultrafast decay component to the ground state in all DNA/RNA nucleobases. Basis set size seems to have a minor effect, appearing to be relevant only for purine-based derivatives. We rule out structural changes as a key factor in classifying the different conical intersections, which display almost identical geometries across active space and basis set change, and we highlight instead the importance of correctly describing the electronic states involved at these crossing points. Our work shows that careful active space selection is essential to accurately describe conical intersection topographies and therefore to adequately account for their active role in molecular photochemistry.
29.	De Vico, Luca, et al. (författare) Chemiluminescence of 1,2-dioxetane. Reaction mechanism uncovered 2007 Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:32, s. 8013-8019 Tidskriftsartikel (refereegranskat)abstract The thermal decomposition of 1,2-dioxetane and the associated production of chemiluminescent products, model for a wide range of chemiluminescent reactions, has been studied at the multistate multiconfigurational second-order perturbation level of theory. This study is in qualitative and quantitative agreement with experimental observations with respect to the activation energy and the observed increase of triplet and singlet excited products as substituents are added to the parent molecule. The, previously incomplete, reaction mechanism of the chemiluminescence of 1,2-dioxetane is now rationalized and described as mainly due to a particular form of entropic trapping.
30.	De Vico, Luca, et al. (författare) New general tools for constrained geometry optimizations 2005 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 1:5, s. 1029-1037 Tidskriftsartikel (refereegranskat)abstract A modification of the constrained geometry optimization method by Anglada and Bofill (Anglada, J. M.; Bofill, J. M. J. Comput. Chem. 1997, 18, 992-1003) is designed and implemented. The changes include the choice of projection, quasi-line-search, and the use of a Rational Function optimization approach rather than a reduced-restricted-quasi-Newton-Raphson method in the optimization step. Furthermore, we show how geometrical constrains can be implemented in an approach based on nonreclunclant curvilinear coordinates avoiding the inclusion of the constraints in the set of redundant coordinates used to define the internal coordinates. The behavior of the new implementation is demonstrated in geometry optimizations featuring single or multiple geometrical constraints (bond lengths, angles, etc.), optimizations on hyperspherical cross sections (as in the computation of steepest descent paths), and location of energy minima on the intersection subspace of two potential energy surfaces (i.e. minimum energy crossing points). In addition, a novel scheme to determine the crossing point geometrically nearest to a given molecular structure is proposed.
31.	Delcey, Mickael G., et al. (författare) Exact semi-classical light-matter interaction operator applied to two-photon processes with strong relativistic effects 2020 Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 153:2 Tidskriftsartikel (refereegranskat)abstract X-ray processes involve interactions with high-energy photons. For these short wavelengths, the perturbing field cannot be treated as constant, and there is a need to go beyond the electric-dipole approximation. The exact semi-classical light-matter interaction operator offers several advantages compared to the multipole expansion such as improved stability and ease of implementation. Here, the exact operator is used to model x-ray scattering in metal K pre-edges. This is a relativistic two-photon process where absorption is dominated by electric-dipole forbidden transitions. With the restricted active space state-interaction approach, spectra can be calculated even for the multiconfigurational wavefunctions including second-order perturbation. However, as the operator itself depends on the transition energy, the cost for evaluating integrals for hundreds of thousands unique transitions becomes a bottleneck. Here, this is solved by calculating the integrals in a molecular-orbital basis that only runs over the active space, combined with a grouping scheme where the operator is the same for close-lying transitions. This speeds up the calculations of single-photon processes and is critical for the modeling of two-photon scattering processes. The new scheme is used to model Kα resonant inelastic x-ray scattering of iron-porphyrin complexes with relevance to studies of heme enzymes, for which the total computational time is reduced by several orders of magnitude with an effect on transition intensities of 0.1% or less.
32.	Devarajan, Ajitha, et al. (författare) Role of Electronic Curve Crossing of Benzene S-1 State in the Photodissociation of Aryl Halides, Effect of Fluorination : RASSI-SO MS-CASPT2 Study 2009 Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 109:9, s. 1962-1974 Tidskriftsartikel (refereegranskat)abstract An ab initio study of the role of electronic curve crossing of benzene S-1 state in the photo dissociation dynamics of the iodobenzene and effect of fluorination is presented. Two dissociative life times observed in iodobenzene is attributed to the coupled repulsive potential energy curves of the low-lying n-sigma, pi-sigma, pi-pi* states. The direct channel is attributed to the alkyl like transition and the indirect channel is attributed to the mixing of the alkyl like transitions with the low lying benzene pi-pi* transitions. Fluorination of iodobenzene results in a substantial increase in the direct channel product. To analyze the possible role of electronic curve crossing of these transitions, potential energy curves of low-lying n-sigma, pi-sigma, pi-pi* states were studied including spin-orbit and relativistic effects using the Restricted Active Space state interaction multistate complete active space perturbation theory (RASSI-MS-CASPT2) method. Crossing behavior of spin-free and spin-orbit potential energy curves was analyzed for the role of the benzene S-1 state. Our results indicate the curve crossing region to be around 2.00-2.35 angstrom for both C6H5I and C6F5I. Analysis of effect of fluorination on the energies of states corresponding to benzene pi-pi* and n-sigma* transitions suggests an increase in the energy of benzene pi-pi* states and a decrease in the energy of the states corresponding to n-sigma* transitions. Increased spin-orbit gap, increased separation of the benzene S-1(pi-pi) state and n-sigma states in the region of curve crossing, lesser mixing of the pi-pi* and n-sigma* states, an order of magnitude decrease in the transition strength to the benzene singlet transition all contributed to the observed Substantial increase in the quantum yield of the direct channel product on fluorination of aryl halides.
33.	Engkvist, Ola, et al. (författare) On the relation between retention indexes and the interaction between the solute and the column in gas-liquid chromatography 1996 Ingår i: Journal of chemical information and computer sciences. - : American Chemical Society (ACS). - 0095-2338 .- 1520-5142. ; 36:6, s. 1153-1161 Tidskriftsartikel (refereegranskat)abstract Gas-liquid chromatography retention indexes for organic molecules are determined by the interaction between the molecule and the column liquid phase. In this article, a model for calculating the interaction energy between a molecule and a dielectric wall is developed. The model is at least to our knowledge the first attempt to predict retention indexes from the interaction between the molecules and the column. This approach to predict retention indexes is radically different from methods proposed before. Earlier predictions of the retention indexes have been done by a large number of descriptors, which were Linearly correlated to the retention indexes. The developed model has been tested for polycyclic aromatic hydrocarbons mainly with a molecular weight of 302. For the molecules with MW 302 the obtained correlation coefficient is 0.92. A somewhat simpler model is used to fit PAH with different MWs. A correlation coefficient of 0.998 is obtained if the retention indexes were fitted to the logarithm of the interaction energies between the PAHs and the column.
34.	Farahani, Pooria, 1985-, et al. (författare) Advances in computationalphotochemistry and chemiluminescenceof biological and nanotechnologicalmolecules 2017. - 44 Ingår i: Photochemistry. - Cambridge, UK : Royal Society of Chemistry. - 1465-1947. ; , s. 16-60, s. 16-60 Bokkapitel (refereegranskat)abstract Recent advances (2014–2015) in computational photochemistry and chemiluminescencederive from the development of theory and from the application of state-of-the-art andnew methodology to challenging electronic-structure problems. Method developmentshave mainly focused, first, on the improvement of approximate and cheap methods toprovide a better description of non-adiabatic processes, second, on the modification ofaccurate methods in order to decrease the computation time and, finally, on dynamicsapproaches able to provide information that can be directly compared with experimentaldata, such as yields and lifetimes. Applications of the ab initio quantum-chemistry methodshave given rise to relevant findings in distinct fields of the excited-state chemistry.We brieflysummarise, in this chapter, the achievements on photochemical mechanisms andchemically-induced excited-state phenomena of interest in biology and nanotechnology.
35.	Fernández Galván, Ignacio, 1977-, et al. (författare) Restricted-Variance Constrained, Reaction Path, and Transition State Molecular Optimizations Using Gradient-Enhanced Kriging 2021 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 17:1, s. 571-582 Tidskriftsartikel (refereegranskat)abstract Gaussian process regression has recently been explored as an alternative to standard surrogate models in molecular equilibrium geometry optimization. In particular, the gradient-enhanced Kriging approach in association with internal coordinates, restricted-variance optimization, and an efficient and fast estimate of hyperparameters has demonstrated performance on par or better than standard methods. In this report, we extend the approach to constrained optimizations and transition states and benchmark it for a set of reactions. We compare the performance of the newly developed method with the standard techniques in the location of transition states and in constrained optimizations, both isolated and in the context of reaction path computation. The results show that the method outperforms the current standard in efficiency as well as in robustness.
36.	Fernández Galván, Ignacio, 1977-, et al. (författare) Smooth Things Come in Threes : A Diabatic Surrogate Model for Conical Intersection Optimization 2023 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 19:11, s. 3418-3427 Tidskriftsartikel (refereegranskat)abstract The optimization of conical intersection structures iscomplicatedby the nondifferentiability of the adiabatic potential energy surfaces.In this work, we build a pseudodiabatic surrogate model, based onGaussian process regression, formed by three smooth and differentiablesurfaces that can adequately reproduce the adiabatic surfaces. Usingthis model with the restricted variance optimization method resultsin a notable decrease of the overall computational effort requiredto obtain minimum energy crossing points.
37.	Francés-Monerris, Antonio, et al. (författare) Triplet versus singlet chemiexcitation mechanism in dioxetanone : a CASSCF/CASPT2 study 2017 Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 136:6 Tidskriftsartikel (refereegranskat)abstract Chemiluminescence is a fundamental process of chemistry consisting in the conversion of chemical energy stored in chemical bonds into light. It is used by nature and by man-made technology, being especially relevant in chemical analysis. The understanding of the phenomenon strongly relies in the study of peroxide models such as 1,2-dioxetanones. In the present contribution, the singlet S2 and the triplet T2 potential energy surfaces of the unimolecular decomposition of 1,2-dioxetanone have been mapped along the O-O and C-C bond coordinates on the grounds of the multiconfigurational CASPT2//CASSCF approach. Results confirm the energy degeneracy between T2, T1, S1, and S0 at the TS region, whereas S2 is unambiguously predicted at higher energies. Triplet-state population is also supported by the spin-orbit couplings between the singlet and triplet states partaking in the process. In particular, the first-principle calculations show that decomposition along the T2 state is a competitive process, having a small (similar to 3 kcal/mol) energy barrier from the ground-state TS structure. The present findings can explain the higher quantum yield of triplet-state population with respect to the excited singlet states recorded experimentally for the uni-molecular decomposition of 1,2-dioxetanone models.
38.	Gaenko, Alexander V, et al. (författare) Ab initio DFT study of Z-E isomerization pathways of N-benzylideneaniline 2007 Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 118:1, s. 271-279 Tidskriftsartikel (refereegranskat)abstract The ground state properties and absorption spectra of N-benzylideneaniline (NBA) have been studied at the density functional (DFT) and at the time-dependent density functional (TD-DFT) level of the theory. The equilibrium geometries of the E and Z isomers in the ground state and their vibrational frequencies have been computed. Furthermore, the excitation energies of the lowest excited singlet and triplet states and the potential energy curves along the torsion and the inversion isomerization coordinates were evaluated. The results are discussed in light of the available experimental data.
39.	Gagliardi, Laura, et al. (författare) Dissociation reaction of N-8 azapentalene to 4N(2) : A theoretical study 2000 Ingår i: International Journal of Quantum Chemistry. - 0020-7608 .- 1097-461X. ; 77:1, s. 311-315 Tidskriftsartikel (refereegranskat)abstract We present a theoretical study on the dissociation reaction of Ng azapantalene to four N-2 molecules. The process proceeds via isomerization of Ns azapentalene to N-8 azidopentazole, which then dissociates directly into four nitrogen molecules. The calculations have determined the relative energies of the two isomers and the two transition states involved in the dissociation process. The results show azidopentazole to be 13 kcal/mol more stable than azapentalene. The barrier to dissociation into four N-2 molecules is computed to be 19 kcal/mol. It is concluded that Ns is not stable enough to be considered as a candidate for a high-energy density material. The calculations have been carried out using multiconfigurational self-consistent field and second-order perturbation theory.
40.	Gagliardi, Laura, et al. (författare) Local properties of quantum chemical systems : The LoProp approach 2004 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 121:10, s. 4494-4500 Tidskriftsartikel (refereegranskat)abstract A new method is presented, which makes it possible to partition molecular properties like multipole moments and polarizabilities, into atomic and interatomic contributions. The method requires a subdivision of the atomic basis set into occupied and virtual basis functions for each atom in the molecular system. The localization procedure is organized into a series of orthogonalizations of the original basis set, which will have as a final result a localized orthonormal basis set. The new localization procedure is demonstrated to be stable with various basis sets, and to provide physically meaningful localized properties. Transferability of the methyl properties for the alkane series and of the carbon and hydrogen properties for the benzene, naphtalene, and anthracene series is demonstrated.
41.	Galván, Ignacio Fdez., et al. (författare) OpenMolcas : From Source Code to Insight 2019 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 15:11, s. 5925-5964 Tidskriftsartikel (refereegranskat)abstract In this Article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the Article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets.
42.	Giussani, Angelo, et al. (författare) Molecular Basis of the Chemiluminescence Mechanism of Luminol 2019 Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 25:20, s. 5202-5213 Tidskriftsartikel (refereegranskat)abstract Light emission from luminol is probably one of the most popular chemiluminescence reactions due to its use in forensic science, and has recently displayed promising applications for the treatment of cancer in deep tissues. The mechanism is, however, very complex and distinct possibilities have been proposed. By efficiently combining DFT and CASPT2 methodologies, the chemiluminescence mechanism has been studied in three steps: 1)luminol oxygenation to generate the chemiluminophore, 2)a chemiexcitation step, and 3)generation of the light emitter. The findings demonstrate that the luminol double-deprotonated dianion activates molecular oxygen, diazaquinone is not formed, and the chemiluminophore is formed through the concerted addition of oxygen and concerted elimination of nitrogen. The peroxide bond, in comparison to other isoelectronic chemical functionalities (-NH-NH-, -N--N--, and -S-S-), is found to have the best chemiexcitation efficiency, which allows the oxygenation requirement to be rationalized and establishes general design principles for the chemiluminescence efficiency. Electron transfer from the aniline ring to the OO bond promotes the excitation process to create an excited state that is not the chemiluminescent species. To produce the light emitter, proton transfer between the amino and carbonyl groups must occur; this requires highly localized vibrational energy during chemiexcitation.
43.	Guo, Meiyuan, et al. (författare) Fingerprinting Electronic Structure of Heme Iron by Ab Initio Modeling of Metal L-Edge X-ray Absorption Spectra 2019 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 15:1, s. 477-489 Tidskriftsartikel (refereegranskat)abstract The capability of the multiconfigurational restricted active space approach to identify electronic structure from spectral fingerprints is explored by applying it to iron L-edge X-ray absorption spectroscopy (XAS) of three heme systems that represent the limiting descriptions of iron in the Fe-O-2 bond, ferrous and ferric [Fe(P)(ImH)(2)](0/1+) (P = porphine, ImH = imidazole), and Fe-II(P). The level of agreement between experimental and simulated spectral shapes is calculated using the cosine similarity, which gives a quantitative and unbiased assignment. Further dimensions in fingerprinting are obtained from the L-edge branching ratio, the integrated absorption intensity, and the edge position. The results show how accurate ab initio simulations of metal L-edge XAS can complement calculations of relative energies to identify unknown species in chemical reactions.
44.	Gusarov, Sergey, et al. (författare) Correlation potentials for a multiconfigurational-based density functional theory with exact exchange 2004 Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 112:2, s. 84-94 Tidskriftsartikel (refereegranskat)abstract A density functional theory based on a complete active space self consistent field (CASSCF) reference function with exact exchange is discussed. It is first shown that such a theory may be formulated with a correlation potential dependent on the density function and on the active space used. Auxiliary functions, such as the on-top two-particle density, are used to define uniquely the potential for different active spaces. The paper also analyses the correlation functional for some atomic and molecular cases. Large ab initio calculations are performed to obtain accurate density functions. A correlation potential is then fitted such that the reference CASSCF function gives the same density. The correlation potential values are saved in a data base for future analysis.
45.	Gusarov, Sergey, et al. (författare) Using on-top pair density for construction of correlation functionals for multideterminant wave functions 2004 Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 102:21-22, s. 2207-2216 Tidskriftsartikel (refereegranskat)abstract The value of the two-particle density function at coalescence P-2(ontop)(r) = rho(2)(r, r) is frequently used as an additional variable for formulating approximate exchange-correlation or correlation functionals. Here, its applications as one of the key variables for the construction of new DFT (preferably multi-determinant) functionals is investigated. The basic formalism is presented and it is shown that this replacement avoids some difficulty to construct a Fock matrix in a ROKS (restricted open-shell Kohn-Sham) method and also to reduce the ‘double counting’ of correlation energy in CASDFT (complete active space density functional theory) calculations. Calculations of excitation energies for transition metals and dissociation curves for diatomic molecules are presented as an example.
46.	Hase, Florian, et al. (författare) Machine learning for analysing ab initio molecular dynamics simulations 2020 Ingår i: 31St International Conference On Photonic, Electronic And Atomic Collisions (Icpeac Xxxi). - : IOP PUBLISHING LTD. Konferensbidrag (refereegranskat)abstract Post-calculation analyses are often required to extract physical insights from ab initio molecular dynamics simulations. In the present work, we use different machine learning classifiers to take a new perspective on the decomposition reaction of dioxetane. Upon thermally activated decomposition, dioxetane can form products in an electronically excited state and can thus chemiluminesce. Simulated dynamics trajectories exhibit both successful and frustrated dissociations. As an exhaustive and systematic study of the decomposition mechanism "by hand" is beyond feasibility, machine learning models have been employed to study the relevant nuclear distortions governing molecular dissociation. According to all classifiers used in the study, the two sets of geometries differ by the in-phase planarisation of the two formaldehyde moieties. New insights are obtained from this analysis: if both moieties are not planar enough when the dissociation is attempted, it is frustrated and the molecule remains trapped. The postponing of the decomposition reaction by the so-called entropic trap enhances the chemiexcitation efficiency.
47.	Head-Gordon, Martin, et al. (författare) A celebration of the Swedish school 2017 Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 115:17-18, s. 1993-1994 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
48.	Hermida-Ramón, Jose Manuel, et al. (författare) Analysis of the relative stability of cis-urocanic acid in condensed phase : The use of Langevin dipoles 2002 Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 106:28, s. 7115-7120 Tidskriftsartikel (refereegranskat)abstract A series of ab initio calculations employing a modified Langevin dipoles method to model neutral, cationic, and anionic cis-urocanic acid in human skin is presented. A comparison between the stability of the conformers in gas phase and in a condensed phase is performed. In particular, the energy barrier and transition state of the isomerization reaction of the anionic forms of cis-urocanic acid have been characterized. The modifications of the Langevin dipoles method, a procedure to obtain the required model parameters, and model verifications are presented. The latter include computing the water solvation energy and the free energy of the water dissociation.
49.	Holt, Asbjörn, et al. (författare) A NEMO potential that includes the dipole-quadrupole and quadrupole-quadrupole polarizability 2010 Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 31:8, s. 1583-1591 Tidskriftsartikel (refereegranskat)abstract To increase the accuracy of molecular force fields a systematical and balanced improvement of the various terms included is needed. In this work, we have followed this strategy to improve the quality of the NEMO potential for the formaldehyde dimer by introducing local quadrupole moments and higher-order polarizabilities. It is found that inclusion of the quadrupole moment significantly improves the interaction potential. Furthermore, the inclusion of higher-order polarizabilities up to quadrupole-quadrupole polarizability is shown to give a better description of the intermolecular interaction. In addition, it is demonstrated that localized properties based on MP2 densities reproduces the BSSE corrected MP2 interaction energy at large intermolecular separations. This is not the case for HF-SCF based properties.
50.	Holt, Asbjörn, et al. (författare) The charge capacity of the chemical bond 2007 Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 436:1-3, s. 297-301 Tidskriftsartikel (refereegranskat)abstract A bond capacitance is introduced via the LoProp formalism, and the calculated bond capacitances reflects the properties of the corresponding chemical bond remarkably well. It is clearly possible to characterize the properties of the chemical bond with the help of the calculated bond capacitances.

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