SwePub - sökning: WFRF:(Lindh Roland)

Numrering	Referens	Omslagsbild	Hitta
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.	Andrews, Lester, et al. (författare) Simple N UF3 and P UF3 molecules with triple bonds to uranium 2008 Ingår i: Angewandte Chemie (International edition). - : Wiley. - 1521-3773 .- 1433-7851. ; 47:29, s. 5366-5370 Tidskriftsartikel (refereegranskat)
4.	Aquilante, Francesco, et al. (författare) Accurate ab initio density fitting for multiconfigurational self-consistent field methods 2008 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 129:2, s. 024113- Tidskriftsartikel (refereegranskat)abstract Using Cholesky decomposition and density fitting to approximate the electron repulsion integrals, an implementation of the complete active space self-consistent field (CASSCF) method suitable for large-scale applications is presented. Sample calculations on benzene, diaquo-tetra-mu-acetato-dicopper(II), and diuraniumendofullerene demonstrate that the Cholesky and density fitting approximations allow larger basis sets and larger systems to be treated at the CASSCF level of theory with controllable accuracy. While strict error control is an inherent property of the Cholesky approximation, errors arising from the density fitting approach are managed by using a recently proposed class of auxiliary basis sets constructed from Cholesky decomposition of the atomic electron repulsion integrals.
5.	Aquilante, Francesco, et al. (författare) Analytic derivatives for the Cholesky representation of the two-electron integrals 2008 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 129:3, s. 034106- Tidskriftsartikel (refereegranskat)abstract We propose a formalism for calculating analytic derivatives of the electronic energy with respect to nuclear coordinates using Cholesky decomposition of the two-electron integrals. The formalism is derived by exploiting the equivalence of Cholesky decomposition and density fitting when a suitable auxiliary basis set is used for expanding atomic orbital product densities in the latter. An implementation of gradients at the nonhybrid density functional theory level is presented, and sample calculations demonstrate that the errors in equilibrium geometries due to the Cholesky representation of the integrals can be controlled by adjusting the decomposition threshold.
6.	Aquilante, Francesco, et al. (författare) Atomic Cholesky decompositions: A route to unbiased auxiliary basis sets for density fitting approximation with tunable accuracy and efficiency 2009 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:15 Tidskriftsartikel (refereegranskat)abstract Cholesky decomposition of the atomic two-electron integral matrix has recently been proposed as a procedure for automated generation of auxiliary basis sets for the density fitting approximation [F. Aquilante , J. Chem. Phys. 127, 114107 (2007)]. In order to increase computational performance while maintaining accuracy, we propose here to reduce the number of primitive Gaussian functions of the contracted auxiliary basis functions by means of a second Cholesky decomposition. Test calculations show that this procedure is most beneficial in conjunction with highly contracted atomic orbital basis sets such as atomic natural orbitals, and that the error resulting from the second decomposition is negligible. We also demonstrate theoretically as well as computationally that the locality of the fitting coefficients can be controlled by means of the decomposition threshold even with the long-ranged Coulomb metric. Cholesky decomposition-based auxiliary basis sets are thus ideally suited for local density fitting approximations.
7.	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.
8.	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.
9.	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.
10.	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.
11.	Aquilante, Francesco, et al. (författare) Molcas 8 : New capabilities for multiconfigurational quantum chemical calculations across the periodic table 2016 Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 37:5, s. 506-541 Tidskriftsartikel (refereegranskat)abstract In this report, we summarize and describe the recent unique updates and additions to the Molcas quantum chemistry program suite as contained in release version 8. These updates include natural and spin orbitals for studies of magnetic properties, local and linear scaling methods for the Douglas-Kroll-Hess transformation, the generalized active space concept in MCSCF methods, a combination of multiconfigurational wave functions with density functional theory in the MC-PDFT method, additional methods for computation of magnetic properties, methods for diabatization, analytical gradients of state average complete active space SCF in association with density fitting, methods for constrained fragment optimization, large-scale parallel multireference configuration interaction including analytic gradients via the interface to the Columbus package, and approximations of the CASPT2 method to be used for computations of large systems. In addition, the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package Cobramm. Further, a module to run molecular dynamics simulations is added, two surface hopping algorithms are included to enable nonadiabatic calculations, and the DQ method for diabatization is added. Finally, we report on the subject of improvements with respects to alternative file options and parallelization.
12.	Aquilante, Francesco, et al. (författare) MOLCAS-a software for multiconfigurational quantum chemistry calculations 2013 Ingår i: Wiley Interdisciplinary Reviews. Computational Molecular Science. - : Wiley. - 1759-0884 .- 1759-0876. ; 3:2, s. 143-149 Tidskriftsartikel (refereegranskat)abstract At variance, with most of the quantum chemistry software presently available, MOLCAS is a package that is specialized in multiconfigurational wave function theory (MC-WFT) rather than density functional theory (DFT). Given the much higher algorithmic complexity of MC-WFT versus DFT, an extraordinary effort needs to be made from the programming point of view to achieve state-of-the-art performance for large-scale calculations. In particular, a robust and efficient implementation of the Cholesky decomposition techniques for handling two-electron integrals has been developed which is unique to MOLCAS. Together with this 'Cholesky infrastructure', a powerful and multilayer graphical and scripting user interface is available, which is an essential ingredient for the setup of MC-WFT calculations. These two aspects of the MOLCAS software constitute the focus of the present report. (C) 2012 John Wiley & Sons, Ltd.
13.	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.
14.	Aquilante, Francesco, et al. (författare) Unbiased auxiliary basis sets for accurate two-electron integral approximations 2007 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127:11, s. 114107- Tidskriftsartikel (refereegranskat)abstract We propose Cholesky decomposition (CD) of the atomic two-electron integral matrix as a robust and general technique for generating auxiliary basis sets for the density fitting approximation. The atomic CD (aCD) auxiliary basis set is calculated on the fly and is not biased toward a particular quantum chemical method. Moreover, the accuracy of the aCD basis set can be controlled with a single parameter.
15.	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.
16.	Atsumi, Michiko, et al. (författare) Ab initio and DFT analysis of the low-lying electronic states of metal dihalides : quantum chemical calculations on the neutral BrMCl (M = Cu, Ag, Au) 2013 Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 15:25, s. 10151-10157 Tidskriftsartikel (refereegranskat)abstract The electronic configuration of the electronic ground and low-lying doublet excited states of neutral metal dihalides BrMCl (M = Cu, Ag, Au) has been investigated on the basis of CASSCF/CASPT2 methods taking into account scalar relativistic effects. A preliminary study of the electronic problem in BrAgCl, based on DFT and CASSCF/CASPT2 approaches and using various basis sets, namely relativistic all-electron basis sets, effective core potentials and ab initio model potentials (AIMP), as well as non-relativistic AIMP is discussed. It is shown that single-determinant methods are not flexible enough to describe the bonding of the neutral species in the electronic ground state regardless of the basis set. The failure to allocate the single electron of BrAgCl correlates with a wrong charge distribution within the complex, which is more accentuated when using pseudopotential basis sets. The inclusion of static and dynamic correlation effects by means of CASSCF/CASPT2 methods using large relativistic all-electron basis sets provides a correct qualitative picture of the electronic structure of the BrMCl series (M = Cu, Ag, Au). The spin unrestricted KS-DFT approach leads to a reasonable description of the degenerate electronic ground state ((2)Sigma/(2)Pi) bonding in these complexes with negligible spin contamination providing comparative spin densities in the series of molecules under investigation.
17.	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.
18.	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.
19.	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.
20.	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.
21.	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.
22.	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.
23.	Barone, Giampaolo, et al. (författare) Nuclear quadrupole moment of Sn-119 2008 Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:7, s. 1666-1672 Tidskriftsartikel (refereegranskat)abstract Second-order scalar-relativistic Douglas-Kroll-Hess density functional calculations of the electric field gradient, including an analytic correction of the picture change error, were performed for 34 tin compounds of which molecular structures and Sn-119 Mossbauer spectroscopy parameters are experimentally known. The components of the diagonalized electric field gradient tensor, V-xx,V-yy, V-zz, were used to determine the quantity V, which is proportional to the nuclear quadrupole splitting parameter Delta E. The slope of the linear correlation plot of the experimentally determined Delta E parameter versus the corresponding calculated V data allowed us to obtain an absolute value of the nuclear quadrupole moment Q of Sn-119 equal to Q = 13.2 +/- 0.1 fm(2). This is about 11% larger than the picture-change-error-affected value and in good agreement with previous estimates of the picture change error in compounds of similar atomic charge. Moreover, despite the variety of the tin compounds considered in this study, the new result is in excellent agreement with the previously determined most accurate value of Q for Sn-119 of Q = 12.8 +/- 0.7 fm(2), but with a noticeably narrower error bar. The reliability of the calibration method in the calculation of the Delta E parameter of tin compounds is within a margin of +/- 0.3 mm s(-1) when compared to experimental data and does not depend on the inclusion of the picture change correction in the density functional calculations but is essentially determined by the use of an atomic natural orbital relativistic core-correlated basis set for the description of the core electron density. The results obtained suggest that the present picture-change-corrected Douglas-Kroll-Hess approach provides reliable electric field gradients in the case of closed-shell metal compounds involving elements up to the fifth row of the periodic table for which spin-orbit coupling is negligible.
24.	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.
25.	Battaglia, Stefano, et al. (författare) Multiconfigurational quantum chemistry: The CASPT2 method 2023 Ingår i: Theoretical and Computational Photochemistry. - Amsterdam : Elsevier. ; , s. 135-162 Bokkapitel (refereegranskat)abstract This chapter presents the theory behind the CASPT2 method and its adaptation to a multistate formalism. The chapter starts with an introduction to the theory of the CASPT2 method—an application of the Rayleigh-Schrödinger perturbation theory applied to the multiconfigurational reference function—as it was originally presented. In particular, we discuss the nature of the reference Hamiltonian and the first-order interacting space. This is followed by some detailed discussion with respect to the intruder state problem and various shift techniques to address this problem. Afterward, a longer review on alternative reference Hamiltonians, which to some degree or completely remove the intruder state problem, is put forward. Subsequently, the presently proposed multistate versions of the CASPT2 method are discussed in some detail. The chapter is concluded with a review of different benchmark assessments of the accuracy of the method and a qualified suggestion on the future development potentials of the approach.
26.	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.
27.	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.
28.	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.
29.	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).
30.	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.
31.	Borne, Kurtis D., et al. (författare) Ultrafast electronic relaxation pathways of the molecular photoswitch quadricyclane 2024 Ingår i: NATURE CHEMISTRY. - : Springer Nature. - 1755-4330 .- 1755-4349. ; 16, s. 499-505 Tidskriftsartikel (refereegranskat)abstract The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.
32.	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.
33.	Boström, Jonas, et al. (författare) Analytical gradients of Hartree-Fock exchange with density fitting approximations 2013 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 9:1, s. 204-212 Tidskriftsartikel (refereegranskat)abstract We extend the local exchange (LK) algorithm [Aquilante, F.; Pedersen, T. B.; Lindh, R. J. Chem. Phys.2007, 126, 194106] to the calculation of analytical gradients with density fitting. We discuss the features of the screening procedure and demonstrate the possible advantages of using this formulation, which is easily interfaced to a standard integral-direct gradient code. With auxiliary basis sets obtained from Cholesky decomposition of atomic or molecular integral blocks with a decomposition threshold of 10-4Eh, typical errors due to the density fitting in bond lengths, bond angles, and dihedral angles are 0.1 pm, 0.1°, and 0.5°, respectively. The overall speedup of geometry optimizations is about 1 order of magnitude for atomic natural-orbital-type basis sets but much less pronounced for correlation-consistent basis sets.
34.	Boström, Jonas, et al. (författare) Analytical gradients of the second-order Møller–Plesset energy using Cholesky decompositions 2014 Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 114:5, s. 321-327 Tidskriftsartikel (refereegranskat)abstract An algorithm for computing analytical gradients of the second-order Møller–Plesset (MP2) energy using density fitting (DF) is presented. The algorithm assumes that the underlying canonical Hartree–Fock reference is obtained with the same auxiliary basis set, which we obtain by Cholesky decomposition (CD) of atomic electron repulsion integrals. CD is also used for the negative semidefinite MP2 amplitude matrix. Test calculations on the weakly interacting dimers of the S22 test set (Jurečka et al., Phys. Chem. Chem. Phys. 2006, 8, 1985) show that the geometry errors due to the auxiliary basis set are negligible. With double-zeta basis sets, the error due to the DF approximation in intermolecular bond lengths is better than 0.1 pm. The computational time is typically reduced by a factor of 6–7.
35.	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.
36.	Boström, Jonas, et al. (författare) Coupled Cluster and Moller-Plesset Perturbation Theory Calculations of Noncovalent Intermolecular Interactions using Density Fitting with Auxiliary Basis Sets from Cholesky Decompositions 2012 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 8:6, s. 1921-1928 Tidskriftsartikel (refereegranskat)abstract We compute noncovalent intermolecular interaction energies for the S22 test set [Phys. Chem. Chem. Phys. 2006, 8, 1985-1993] of molecules at the Moller-Plesset and coupled cluster levels of supermolecular theory using density fitting (DF) to approximate all two-electron integrals. The error due to the DF approximation is analyzed for a range of auxiliary basis sets derived from Cholesky decomposition (CD) in conjunction with correlation consistent and atomic natural orbital valence basis sets. A Cholesky decomposition threshold of 10(-4)E(h) for full molecular CD and its one-center approximation (1C-CD) generally yields errors below 0.03 kcal/mol, whereas 10(-3)E(h) is sufficient to obtain the same level of accuracy or better with the atomic CD (aCD) and atomic compact CD (acCD) auxiliary basis sets. Comparing to commonly used predefined auxiliary basis sets, we find that while the aCD and acCD sets are larger by a factor of 2-4 with triple-zeta AO basis sets, they provide results 1-2 orders of magnitude more accurate.
37.	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.
38.	Chen, Shufeng, et al. (författare) Bioluminescence of Obelin : identification of the light emitters using QM/MM models 2014 Ingår i: Luminescence (Chichester, England Print). - 1522-7235 .- 1522-7243. ; 29, s. 19-19 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
39.	Chen, Shu-Feng, et al. (författare) Chemiluminescence of Coelenterazine and Fluorescence of Coelenteramide : A Systematic Theoretical Study 2012 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 8:8, s. 2796-2807 Tidskriftsartikel (refereegranskat)abstract A systematic investigation of the structural and spectroscopic properties of coelenteramide has been performed at the TD-CAM-B3LYP/6-31+G(d,p) level of theory, including various fluorescence and chemiluminescence states. The influence of geometric conformations, solvent polarity, protonation state, and the covalent character of the O-H bond of the hydroxyphenyl moiety were carefully studied. Striking differences in geometries and electronic structures among the states responsible for light emission were characterized. All fluorescence states can be described as a limited charge transfer process for a planar amide moiety. However, the chemiluminescence state is characterized by a much larger charge transfer that takes place over a longer distance. Moreover, the chemiluminescent coelenteramide structure exhibits an amide moiety that is no longer planar, in agreement with recent, more accurate ab initio results [Roca-Sanjuan et al J. Chem. Theory Comput. 2011, 7, 4060] Because the chemiluminescence state appears to be completely dark, a new mechanism is tentatively introduced for this process.
40.	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)
41.	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.
42.	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-.
43.	Chen, Shufeng, et al. (författare) Hybrid QM/MM Simulations of the Obelin Bioluminescence and Fluorescence Reveal an Unexpected Light Emitter 2014 Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 118:11, s. 2896-2903 Tidskriftsartikel (refereegranskat)abstract Obelia longissima, a tiny hydrozoan living in temperate and cold seas, features the Obelin photoprotein, which emits blue light. The Obelin bioluminescence and the Ca2+-discharged Obelin fluorescence spectra show multimodal characteristics that are currently interpreted by the concomitant participation of several light emitters. Up to now, the coelenteramide luminophore is thought to exist in different protonation states, one of them engaged in an ion-pair with the nearby residue, His22. Using hybrid quantum mechanics/molecular mechanics (QM/MM) calculations, we demonstrate that such an ion-pair cannot exist as a stable light emitter. However, when His22 electric neutrality is maintained by means of another proton transfer, the phenolate state of coelenteramide exhibits emission properties in agreement with experiment. Finally, an alternative nonradiative decay pathway, involving the formation of a diradical excited state, is postulated for the first time.
44.	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.
45.	Chwee, Tsz S, et al. (författare) Linear scaling multireference singles and doubles configuration interaction 2008 Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128:22, s. 224106- Tidskriftsartikel (refereegranskat)abstract A linear scaling multireference singles and doubles configuration interaction (MRSDCI) method has been developed. By using localized bases to span the occupied and virtual subspace, local truncation schemes can be applied in tandem with integral screening to reduce the various bottlenecks in a MRSDCI calculation. Among these, the evaluation of electron repulsion integrals and their subsequent transformation, together with the diagonalization of the large CI Hamiltonian matrix, correspond to the most computationally intensive steps in a MRSDCI calculation. We show that linear scaling is possible within each step. The scaling of the method with system size is explored with a system of linear alkane chains and we proceed to demonstrate this method can produce smooth potential energy surfaces via calculating the dissociation of trans-6-dodecene (C12H24) along the central C=C bond.
46.	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.
47.	de Jong, Wibe A, et al. (författare) Utilizing high performance computing for chemistry : parallel computational chemistry 2010 Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 12:26, s. 6896-6920 Tidskriftsartikel (refereegranskat)abstract Parallel hardware has become readily available to the computational chemistry research community. This perspective will review the current state of parallel computational chemistry software utilizing high-performance parallel computing platforms. Hardware and software trends and their effect on quantum chemistry methodologies, algorithms, and software development will also be discussed.
48.	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.
49.	De Vico, Luca, et al. (författare) Location of Two Seams in the Proximity of the C-2v pi pi Minimum Energy Path of Formaldehyde 2009 Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 5:1, s. 186-191 Tidskriftsartikel (refereegranskat)abstract Photochemical reactions rationalization is a key aspect for the understanding and setup of novel experiment and novel photoinitiated pathways. In this respect, the relationship between minimum energy paths over an excited-state and the intersection to lower potential energy surfaces is fundamental. In order to help the understanding of this relationship, in this study we present a novel kind of constraint for geometry optimizations, namely, an “orthogonality” constraint. Its possible applications are described. A complete example on how to retrieve the direct relationship between a minimum energy path over an excited-state potential energy surface and a conical intersection seam is given for C-2v symmetry constrained formaldehyde. The advantages of using the novel constraint when rationalizing a (photo)chemical reaction are presented.
50.	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.

Skapa referenser, mejla, bekava och länka

Länka till träfflistan

Träfflista för sökning "WFRF:(Lindh Roland) "

Avgränsa träffmängd

År