| 1. |
- Azizi, Zahra, et al.
(författare)
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How accurate is the CASPT2 method?
- 2006
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 8:23, s. 2727-2732
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Tidskriftsartikel (refereegranskat)abstract
- This paper is a reply to a recent paper on the isomers of trisulfur oxides (Chem. Commun., 2005, 3712) where the accuracy of the CASPT2 method is called in question. We illustrate that their results are due to an inadequate choice of active orbitals. Some results for related O and S containing molecules are also presented to strengthen the argumentation. It is concluded that the CASPT2 method is accurate to at least 0.1 eV (2 kcal mol(-1)) for this type of systems.
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| 2. |
- Kocherbitov, Vitaly, et al.
(författare)
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Hydration of trimethylamine-N-oxide and of dimethyldodecylamine-N-oxide: An ab initio study
- 2007
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Ingår i: Journal of molecular structure. Theochem. - : Elsevier BV. - 0166-1280. ; 808:1-3, s. 111-118
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory (B3LYP) calculations of the hydration of trimethylamine-N-oxide (TMAO) and a surfactant dimethyldodecylamine-N-oxide (DDAO) are reported. Hydrogen-bonded complexes of DDAO with up to three water molecules and of TMAO with up to five water molecules are studied. Interactions of both TMAO and DDAO with water give rise to the binding energies that are much higher than that in the water dimer. A significant charge transfer from amine oxide to water is observed. Energies of water-water interactions increase in the presence of amine oxides. The hydration number of TMAO in the hydrogen bonded complexes is shown to be three. The energies of amine oxide-water interactions in the hydrogen bonded complexes have been analyzed using differential energetic parameters analogous to the parameters measured in calorimetric experiments.
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| 3. |
- La Macchia, Giovanni, et al.
(författare)
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Bond Length and Bond Order in One of the Shortest Cr-Cr Bonds
- 2008
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 47:24, s. 11455-11457
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Tidskriftsartikel (refereegranskat)abstract
- Multiconfigurational quantum chemical calculations on the R-diimines dichromium compound confirm that the Cr-Cr bond, 1.80 angstrom, is among the shortest Cr-I-Cr-I bonds. However, the bond between the two Cr atoms is only a quadruple bond rather than a quintuple bond. The reason why the bond is so short has to be attributed to the strain in the NCCN ligand moieties.
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| 4. |
- Masso, H., et al.
(författare)
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Ab initio characterization of C-5
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127:15
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the structure and spectroscopic parameters of the C-5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea , Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C-5 has a X (1)Sigma(+)(g) linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the nu(7)(pi(u)) anharmonic fundamental has been calculated to be at 102 cm(-1), which validates the assignment to C-5 of the pattern of bands centered at 102 cm(-1) observed with the ISO telescope. (C) 2007 American Institute of Physics.
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| 5. |
- Rocha-Rinza, Tomas, et al.
(författare)
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A theoretical study of singlet low-energy excited states of the benzene dimer
- 2006
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 426:4-6, s. 268-272
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Tidskriftsartikel (refereegranskat)abstract
- The singlet ground and low-energy excited states of the benzene dimer in D-6h geometry are characterized using second order multireference perturbation theory (CASPT2). The counterpoise-corrected spectroscopic parameters of the benzene excimer are in quantitative agreement with experiment. The same method was used to compute a submanifold of the potential energy hypersurface of the first excited state of the benzene dimer, exhibiting two local minima in addition to a saddle point between them. This study demonstrates the suitability of CASPT2 to describe the benzene excimer and suggests that the method can be used to describe weak intermolecular interactions involving excited states. (c) 2006 Elsevier B.V. All rights reserved.
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| 6. |
- Roos, Björn, et al.
(författare)
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New relativistic ANO basis sets for actinide atoms
- 2005
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 409:4-6, s. 295-299
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Tidskriftsartikel (refereegranskat)abstract
- New basis sets of the atomic natural orbital (ANO) type have been developed for the actinide atoms Ac-Cm. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in a electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The lowest multiplet levels have been computed. These calculations include spin-orbit coupling using a variation-perturbation approach. The atomic polarizability of the spherically symmetric americium atom has been computed to be 116 au(3).
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| 7. |
- Roos, Björn, et al.
(författare)
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New relativistic ANO basis sets for transition metal atoms
- 2005
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 109:29, s. 6575-6579
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Tidskriftsartikel (refereegranskat)abstract
- New basis sets of the atomic natural orbital (ANO) type have been developed for the first, second, and third row transition metal atoms. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and polarizabilities for spherically symmetric atoms. These calculations include spin-orbit coupling using a variation-perturbation approach. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The accuracy of computed electron affinities is the same except in cases where the experimental values are smaller than 0.5 eV. Accurate results are obtained for the polarizabilities of atoms with spherical symmetry. Multiplet levels are presented for some of the third row transition metals.
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| 8. |
- Roos, Björn, et al.
(författare)
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New Relativistic Atomic Natural Orbital Basis Sets for Lanthanide Atoms with Applications to the Ce Diatom and LuF3
- 2008
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:45, s. 11431-11435
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Tidskriftsartikel (refereegranskat)abstract
- New basis sets of the atomic natural orbital (ANO) type have been developed for the lanthanide atoms La-Lu. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.1 eV in most cases. Two molecular applications are inluded as illustration: the cerium diatom and the LuF3 molecule. In both cases it is shown that 4f orbitals are not involved in the chemical bond in contrast to an earlier claim for the latter molecule.
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| 9. |
- Roos, Björn, et al.
(författare)
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Not Innocent: Verdict from Ab Initio Multiconfigurational Second-Order Perturbation Theory on the Electronic Structure of Chloroiron Corrole
- 2008
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 112:45, s. 14099-14102
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Tidskriftsartikel (refereegranskat)abstract
- From a suitably broad perspective, transition metal corroles may be viewed as stable, synthetic analogues of high-valent heme protein intermediates such as compounds I and II. Against this backdrop, the electronic structure of chloroiron corrole has provoked a lively debate in recent years. Thus, whereas NMR spectroscopy and DFT calculations suggest an S = 3/2 Fe(III) corrole(center dot 2-) radical description, certain researchers have favored an Fe(IV) formulation. These two descriptions are indistinguishable as far as DFT calculations are concerned. Ab initio CASSCF/CASPT2 calculations provide unambiguous support for the former description. In addition, they rule out any Fe(IV) state, whether high- or low-spin, within 1.5 eV of the ground state.
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| 10. |
- Wang, Xuefeng, et al.
(författare)
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A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N MX3 and P MX3 molecules
- 2008
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:35, s. 8030-8037
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Tidskriftsartikel (refereegranskat)abstract
- Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF3, PF3, and PCl3 to form the simple lowest energy N MF3, and P MX3 products following insertion and halogen transfer, with the exception of P CrF3, which is a higher energy species and is not observed. The E MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.
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