| 1. |
- Al-Abdalla, A, et al.
(författare)
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Ab initio model potential embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6
- 1998
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Ingår i: Journal of Chemical Physics. - : Springer Science and Business Media LLC. - 0021-9606 .- 1089-7690. ; 108, s. 2005-2014
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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.
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| 2. |
- Bernhardsson, Anders, et al.
(författare)
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A direct implementation of the second-order derivatives of multiconfigurational SCF energies and an analysis of the preconditioning in the associated response equation
- 1999
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Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 96:4, s. 617-628
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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.
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| 3. |
- Bernhardsson, Anders, et al.
(författare)
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Direct self-consistent reaction field with Pauli repulsion : Solvation effects on methylene peroxide
- 1996
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 251:3-4, s. 141-149
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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).
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| 4. |
- Choe, Yong-Kee, et al.
(författare)
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Theoretical study of the electronic ground state of iron(II) porphine. II
- 1999
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 111:9, s. 3837-3845
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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.
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| 5. |
- Engkvist, Ola, et al.
(författare)
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On the relation between retention indexes and the interaction between the solute and the column in gas-liquid chromatography
- 1996
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Ingår i: Journal of chemical information and computer sciences. - : American Chemical Society (ACS). - 0095-2338 .- 1520-5142. ; 36:6, s. 1153-1161
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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.
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| 6. |
- Lindh, Roland, 1958-, et al.
(författare)
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Benzyne thermochemistry : A benchmark ab initio study
- 1999
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 103:48, s. 9913-9920
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Tidskriftsartikel (refereegranskat)abstract
- The thermochemistry of the benzynes has been reinvestigated in a set of benchmark calculations including, e.g., multireference perturbation theory in combination with large basis sets up to correlation consistent polarized valence quadruple xi followed by basis set extrapolation procedures. The vibrational corrections have been deduced from multiconfiguration self-consistent field (MCSCF) calculations employing average atomic natural orbital basis sets. The quality of the isodesmic reactions has been investigated by analyzing the errors of the utilized methods in predicting the CH bond strengths and energies of the related molecules. It turns out that multireference third order perturbation theory, although occasionally better than second order, suffers from less systematic errors and thus is not as well suited for use in isodesmic reactions as the corresponding second order theory. The present extended calculations show that all the isodesmic reactions used in previous studies of the thermochemistry of the benzynes are adequate. Furthermore, it is demonstrated that multireference second order perturbation theory accurately reproduces the singlet-triplet energy splittings of the benzynes.
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| 7. |
- Lindh, Roland, 1958-, et al.
(författare)
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Force-constant weighted redundant coordinates in molecular geometry optimizations
- 1999
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 303:5-6, s. 567-575
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Tidskriftsartikel (refereegranskat)abstract
- A novel procedure to select internal coordinates for molecular geometry optimizations is presented. The procedure has features in common with other so-called redundant internal coordinates schemes. It is a black-box method which automatically selects an appropriate set of internal coordinates in which the geometry optimization is performed. The method is explicitly expressed in the non-redundant parameter space, thus avoiding the need for projections from the redundant internal coordinate space. The new procedure introduces a weighting in which the redundancy is modified prior to the generation of the non-redundant internal coordinates. The new method favors those redundant internal coordinates which are the most significant. It has favorable properties for the automatic generation of molecular coordinates in van der Waals complexes and transition state optimizations.
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| 8. |
- Lindh, Roland, 1958-, et al.
(författare)
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On the thermodynamic stability of ArO4
- 1999
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 103:41, s. 8295-8302
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Tidskriftsartikel (refereegranskat)abstract
- The argon tetroxide molecule, ArO4, and the isoelectronically associated perchlorate, ClO4-, and sulfate, SO42-, ions are investigated on different levels of ab initio theory. The equilibrium structures, harmonic vibrational frequencies, and heats of formation are computed applying density functional theory, second order Moller-Plesset perturbation, singles and doubles coupled-cluster with triples corrections, and Bruekner’s doubles coupled-cluster with triples corrections methods in conjunction with various one-particle basis sets. The calculations demonstrate that the description of the bond characteristics in argon tetroxide is sensitive to the applied level of theory. A careful analysis of the global potential energy surface shows that a stationary point exists for the ArO4 complex corresponding to a local mininium. The calculated equilibrium Ar-O bond distance of 1.48 Angstrom for this structure is slightly longer than the corresponding bond length of the perchlorate ion. Harmonic frequencies for ArO4 obtained using Bruekner’s doubles coupled-cluster with triples corrections are found to have a similar pattern like those obtained for the isoelectronic series of ions SiO44-, PO43-, SO42-, ClO4-. Using the concept of an isodesmic reaction, the enthalpy of formation of ArO4 is determined to be endothermic by as much as 1236 kJ/mol. The present theoretically predicted strong endothermicity and the large Ar-O bond distance are in conflict with the monotonic trends obtained for the isoelectronic ions, but can be supported by other chemical extrapolation schemes.
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| 9. |
- Lindh, Roland, 1958-, et al.
(författare)
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On the use of a Hessian model function in molecular geometry optimizations
- 1995
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 241:4, s. 423-428
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Tidskriftsartikel (refereegranskat)abstract
- When a molecular equilibrium geometry is determined by minimizing the energy by a quasi-Newton-Raphson method, the number of iterations required depends critically on the choice of an approximate molecular Hessian matrix. We find that a simple 15-parameter function of the nuclear positions gives a good choice for any molecule with atoms from the first three rows of the periodic table. This Hessian is used for ah initio geometry optimizations with the quasi-Newton-Raphson method, with or without update. The equilibrium geometries of 30 molecules, with a variety of sizes and symmetries, is obtained with the new scheme, which is shown to converge significantly faster than other methods.
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| 10. |
- Lindh, Roland, 1958-, et al.
(författare)
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Research Article Extended ab Initio and Theoretical Thermodynamics Studies of the Bergman Reaction and the Energy Splitting of the Singlet o-, m-, and p-Benzynes
- 1995
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 117:27, s. 7186-7194
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Tidskriftsartikel (refereegranskat)abstract
- The autoaromatization of (Z)-hex-3-ene-1,5-diyne to the singlet biradical p-benzyne has been reinvestigated by state of the art ab initio methods. Previous CCSD(T)/6-31G(d,p) and CASPT2[0]/ANO[C(5s4p2dlf)/H(3s2p)] calculations estimated the reaction heat at 298 K to be 8-10 and 3.9 +/- 3.2 kcal/mol, respectively. Recent NO- and oxygen-dependent trapping experiments and collision-induced dissociation threshold energy experiments estimate the heat of reaction to be 8.5 +/- 1.0 kcal/mol at 470 K (corrected to 9.5 +/- 1.0 kcal/mol at 298 K) and 8.4 +/- 3.0 kcal/mol at 298 K, respectively. New theoretical estimates at 298 K predict the values at the basis set Limit for the CCSD(T) and CASPT2[gl] methods to be 12.7 +/- 2.0 and 5.4 +/- 2.0 kcal/mol, respectively. The experimentally predicted electronic contribution to the heat of activation is 28.6 kcal/mol. This can be compared with 25.5 and 29.8 kcal/mol from the CASPT2[gl] and the CCSD(T) methods, respectively. The new study has a much larger one-particle basis set for the CCSD(T) method as compared to earlier studies. For the CASPT2 investigation the better suited CASPT2[gl] approximation is utilized. The original CASPT2 method, CASPT2[0], systematically favors open-shell systems relative to closed-shell systems. This was previously corrected empirically. The current study shows that the energy difference between CCSD(T) and CASPT2[gl] at the basis set limit is estimated to be 7 +/- 2 kcal/mol. The study also demonstrates that the estimated heat of reaction is very sensitive to the quality of the basis set. In particular CCSD(T)/6-31G(d,p) approach underestimates the basis set limit of the enthalpy by approximately 5 kcal/mol. Furthermore, the relative energies of the p-, m-, and o-benzynes are computed at the CASPT2[gl] and CCSD(T) levels of theory. These results help to explain the discrepancy between the two methods in the case of the Bergman reaction. The deficiency of the CASPT2 method is mainly attributed to the approximate way in which the dynamic correlation is included by perturbation theory. A similar sized error is attributed to the CCSD(T) method due to the approximate way in which near degeneracy effects are included. This combined CCSD(T) and CASPT2[gl] study indicates that the most recent experimental value of the p-benzyne-o-benzyne energy splitting is overestimated.
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| 11. |
- Lindh, Roland, 1958-, et al.
(författare)
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Singlet benzyne thermochemistry : A CASPT2 study of the enthalpies of formation
- 1996
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 258:3-4, s. 409-415
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Tidskriftsartikel (refereegranskat)abstract
- The enthalpies of formation for singlet benzynes were examined at the CASPT2 (second order perturbation theory, based on a complete active space SCF reference function) level of theory, The study employed isodesmic reactions in order to minimize the influence of systematic errors of the method. The results obtained for m- and p-benzyne were found to dispute earlier experimentally deduced values; a discrepancy of about 5 kcal/mol was observed. As a consequence, the enthalpy of reaction of the so called Bergman reaction, deduced previously from the experimental enthalpy of formation of p-benzyne, seems to be overestimated by about 5 kcal/mol. Furthermore, it was observed that the CCSD(T) method (coupled cluster singles and doubles with a perturbatative estimate of the triples contribution) is inappropriate for a balanced treatment of the Bergman reaction, due to the extensive differences in the electronic structures of the two reactants.
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| 12. |
- Moriarty, Niger W, et al.
(författare)
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Tetramethylene : A CASPT2 study
- 1998
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 289:5-6, s. 442-450
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Tidskriftsartikel (refereegranskat)abstract
- The potential energy surface (PES) of the 1,4-tetramethylene biradical has been reinvestigated at the complete-active-space self-consistent field (CASSCF), complete active space with second-order perturbation theory correction and multi-reference configuration interaction (MRCI) levels of theory. The choice of basis set, the size inconsistency of the MRCI wavefunction and the selection of the active space of the CASSCF wavefunction each have a considerable influence on the shape of the PES. In particular, the relative energy of the so-called GF and TF structures depends significantly on the level of theory employed. The results of the study suggest that the experimentally observed intermediate is more likely to be due to entropic trapping rather than the presence of a transition state.
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| 13. |
- Nakayama, Kenichi, et al.
(författare)
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Different bases for different correlation effects : multireference Moller-Plesset perturbation theory in the extended basis function space
- 1999
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 300:3-4, s. 303-311
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Tidskriftsartikel (refereegranskat)abstract
- A method to improve virtual orbitals of CASSCF theory has been proposed by expanding the virtual orbital space through the addition of basis functions without disturbing already established CASSCF function space. It is a natural extension of work used in Hartree-Fock calculations. The improved virtual orbitals are then used to estimate the dynamical correlation. That is, a moderate basis set is used in CASSCF to include nondynamical correlation and a sufficiently large basis set is used in multireference Moller-Plesset perturbation theory to estimate the slowly convergent dynamical correlation. The present formalism is tested on the barrier height of the H2CO –> H-2 + CO reaction, singlet-triplet energy separation in CH2 and valence pi-pi* excitation energies in butadiene. Numerical results show that good accuracy can be obtained.
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| 14. |
- Schütz, Martin, et al.
(författare)
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An integral direct, distributed-data, parallel MP2 algorithm
- 1997
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Ingår i: THEORETICA CHIMICA ACTA. - 0040-5744. ; 95:1-2, s. 13-34
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Tidskriftsartikel (refereegranskat)abstract
- A scalable integral direct, distributed-data parallel algorithm for four-index transformation is presented. The algorithm was implemented in the context of the second-order Moller-Plesset (MP2) energy evaluation, yet it is easily adopted for other electron correlation methods, where only MO integrals with two indices in the virtual orbitals space are required. The major computational steps of the MP2 energy are the two-electron integral evaluation O(N-4) and transformation into the MO basis O(ON4) where N is the number of basis functions, and O the number of occupied orbitals, respectively. The associated maximal communication costs scale as O(n(Sigma)O(2)V N), where V and n(Sigma) denote the number of virtual orbitals, and the number of symmetry-unique shells. The largest local and global memory requirements are Co(N-2) for the MO coefficients and O(OV N) for the three-quarter transformed integrals, respectively. Several aspects of the implementation such as symmetry-treatment, integral prescreening, and the distribution of data and computational tasks are discussed. The parallel efficiency of the algorithm is demonstrated by calculations on the phenanthrene molecule, with 762 primitive Gaussians, contracted to 412 basis functions. The calculations were performed on an IBM SP2 with 48 nodes. The measured wall clock time on 48 nodes is less than 15 min for this calculation, and the speedup relative to single-node execution is estimated to 527. This superlinear speedup is a result of exploiting both the compute power and the aggregate memory of the parallel computer. The tatter reduces the number of passes through the AO integral list, and hence the operation count of the calculation. The test calculations also show that the evaluation of the two-electron integrals dominates the calculation, despite the higher scaling of the transformation step.
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| 15. |
- Schütz, Martin, et al.
(författare)
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Integral-direct electron correlation methods
- 1999
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Ingår i: Molecular Physics. - 0026-8976 .- 1362-3028. ; 96:4, s. 719-733
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Tidskriftsartikel (refereegranskat)abstract
- Integral-direct implementations of Moller-Plesset perturbation theory up to fourth order (MP4(SDQ) without triple excitations), quadratic configuration interaction with single and double excitations (QCISD), coupled cluster (CCSD), multiconfiguration self-consistent field (MCSCF), second- and third-order multireference perturbation theory (MRPT2, MRPT3), and internally contracted multireference configuration interaction (MRCI) methods are described. All methods use the same three integral-direct kernels, namely direct calculation of Fock matrices, direct partial integral transformation, and direct evaluation of external exchange matrices. New algorithms for the latter two procedures are proposed and implemented, for which the memory requirement scales only linearly with basis set size. Test calculations with more than 500 basis functions are presented. It is demonstrated that integral prescreening can significantly reduce the scaling of the computational cost with molecular size N. For small polyglycine peptides (Gly)(n) (n = 1-4), we observe a scaling of N-3 for the direct integral transformation (formal scaling N-5), and N-4 for the direct computation of the external exchange matrices (formal scaling N-6).
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| 16. |
- Schütz, Martin, et al.
(författare)
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The water dimer interaction energy : Convergence to the basis set limit at the correlated level
- 1997
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 107:12, s. 4597-4605
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Tidskriftsartikel (refereegranskat)abstract
- The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Moller-Plesset perturbation theory (MP2). ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (l=7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to -4.94+/-0.02 kcal/mol, with a contribution due to core correlation of -0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set.
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| 17. |
- Serrando-Andrés, Luis, et al.
(författare)
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Theoretical Study of the Internal Charge Transfer in Aminobenzonitriles
- 1995
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 117:11, s. 3189-3204
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Forskningsöversikt (refereegranskat)abstract
- The lower excited states for the molecules aminobenzonitrile (ABN) and (dimethylamino)benzonitrile (DMABN) have been studied as a function of the twisting and wagging motion of the amino group. Theoretical calculations have been performed using the complete active space (GAS) SCF method in combination with multiconfigurational second Order perturbation theory (CASPT2): Basis sets of the ANO-type (C,N/3s2p1d and H/2s) were employed. Ground state geometries were optimized at the CASSCF level. The excitation energies were computed as function of a twist angle, where the amino group is rotated with respect to the benzonitrile plane, and for two values of the wagging angle (0 and 21 degrees). The influence of the wagging angle in the nontwisted molecules was also analyzed. The results fully confirm the twist intramolecular charge transfer (TICT) model proposed to explain the dual fluorescence phenomena occurring in DMABN. The absence of the low frequency part of the fluorescence spectrum in ABN in explained by the shape of the potential energy surface along the isomerization path, due to the large energy gaps among the interacting states, which prevents the amino group from rotating into the TICT state. Calculated transition energies (absorption and emission), structural, and electrical properties of the ground and excited states are in agreement with available experimental information.
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