Quantum Theory of Atoms and Molecules

Si̇nanoğlu, Oktay; Tuan, Debbie Fu-Tai

doi:10.1146/annurev.pc.15.100164.001343

Cited by 66 publications

(63 citation statements)

References 56 publications

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“…This needs to be done using some physical or theoretical model. Two fundamental ideas will guide us in improving the treatment of electron correlation beyond CASSCF(4,4) for these systems: the first is the notion of correlating orbital pairs, or oscillator orbitals in Boys' terminology [29,30], which will assist in the choice of orbitals to include in the active space, and the second is the notion of semi-internal correlation, first discussed by Sinanoglu [31,32] for atoms, which will determine the order to which the configuration interaction expansion should be extended. (For other ways of choosing a RASSCF active space, see [33] and [34].…”

Section: Introductionmentioning

confidence: 99%

Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states

et al. 2015

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(2015) Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states, Molecular Physics, 113:13-14, 1978-1990, DOI: 10.1080/00268976.2015 Physics, 2015 Vol. 113, Nos. 13-14, 1978-1990 The balanced description of ionic and covalent molecular excited electronic states still presents a challenge for current electronic structure methods. In this contribution, we show how the restricted active space self-consistent field (RASSCF) method can be used to address this problem, applied to two dienes in the cis conformation. As with the closely related complete active space self-consistent field (CASSCF) method, the construction of the orbital active space in the RASSCF methodology requires the a priori formulation of a physical or theoretical model of the system being studied. In this article, we discuss how the active space can be constructed in a guided and systematic way, using pairs of natural bond orbitals as correlating partner orbitals (oscillator orbitals) and semi-internal correlation. The resulting balanced description of the covalent and ionic valence excited states -with the ionic state correctly lower in energy at the Franck-Condon geometryis suitable to study the photochemistry of these and other molecules.

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Section: Introductionmentioning

confidence: 99%

Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states

et al. 2015

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“…The justification for the scaling, for which we suggested the name "scaled dynamical correlation" or SDC, is very similar to that for the "scaled external correlation" or SEC method of Brown and Truhlar: 4 the dynamical correlation 5 has a weak geometry dependence and can be scaled by a simple constant factor in order to increase the accuracy of calculations. The difference between the SEC and SDC methods has to do with how the dynamical correlation energy is estimated.…”

Section: Introductionmentioning

confidence: 99%

“…It is recognized that E corr is a composite quantity made up of at least two components: the "dynamical" or "external" correlation energy and the "non-dynamical" or "internal" correlation energy. 5 The SEC method is based on the approximate separation between the internal and external correlation energy afforded by the combination of CASSCF and MR-CI when used with a "sufficiently large" basis set. The CAS energy is expected to include most of the nondynamical correlation while the CI calculation using the CAS solutions as references recovers a portion of the remaining (presumably mostly dynamical) correlation energy.…”

Section: Introductionmentioning

confidence: 99%

Scaled Density Functional Theory Correlation Functionals

Ghouri¹,

Singh²,

Ramachandran³

2007

J. Phys. Chem. A

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We show that a simple one parameter scaling of the dynamical correlation energy estimated by the Density Functional Theory (DFT) correlation functionals helps increase the overall accuracy for several local and nonlocal functionals. The approach taken here has been described as the "scaled dynamical correlation" (SDC) method [B. Ramachandran, J. Phys. Chem. A 2006, 110, 396], and its justification is the same as that of the Scaled External Correlation (SEC) method of Brown and Truhlar. We examine five local and five nonlocal (hybrid) DFT functionals, the latter group including three functionals developed specifically for kinetics by the Truhlar group.The optimum scale factors are obtained using a set of 98 data values consisting of molecules, ions, and transition states. The optimum scale factors, found using a linear regression relationship are found to differ from unity with a high degree of correlation in nearly every case, indicating that the deviation of calculated results from the experimental values are systematic, and proportional to the dynamic correlation energy. As a consequence, the SDC scaling of dynamical correlation decreases the mean errors (signed and unsigned) by significant amounts in an overwhelming majority of cases. These results indicate that there are gains to be realized from further parameterization of several popular exchange-correlation functionals.

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“…It is known indeed that wave functions built from suitable bond orbitals-i.e., orbitals a priori localized on chemical bonds and lone pairs and orthogonalized-give a determinant close to the SCF one [3,4,. Moreover, the use of localized orbitals improves the calculation of the correlation energy and the convergence of the CI treatment [12,13]. The combined use of a perturbation development and bond orbitals gives an expression of the total energy in an additive form, each term of the series presenting a clear physicochemical meaning in terms of bonds and lone pairs interactions.…”

Section: Introductionmentioning

confidence: 99%

Perturbative configuration interaction using localized orbitals in the INDO hypothesis. I. Theory and applications to energetical problems

Douady

Barone

Ellinger

et al. 1980

Int J of Quantum Chemistry

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AbstractsA complete development of the PCILO method is presented for the INDO approximation. The introduction of exchange terms in the theoretical treatment is discussed and a detailed analysis of the successive contributions to the total energy is given. The method is applied to various conformational problems (geometry optimization, rotation, and inversion barriers). Its ability to deal with biological systems is tested on the acetylcholine neurotransmitter. Inversion barriers are highly improved with respect to the corresponding CNDO algorithm. A better account of u -r exchange phenomena can be expected with the presented PCILO-INDO development. Die PcILo-Methode wird im Rahmen der INDO-Naherung entwickelt. Die Einfuhrung von Austauschgliedern in der theoretischen Behandlung wird diskutiert, und eine detaillierte Analyse der sukzessiven Beitrage zur Gesamtenergie wird gegeben. Diese Methode wird auf eine Reihe von Konformationsstudien (Geometrieoptimisierung, Drehungs-, und Inversionsschwellen) angewandt. Ihre Anwendung fur Systeme von biologischem Interesse wird mit dem Neurotransmittor Acetylcholin illustriert. Die Inversionsschwellen werden im Vergleich mit den ursprunglicpn CNDOErgebnissen betrachtlich verbessert. Man kann auch erwarten, dass diese Methode den u-TAustausch besser beschreiben wird.

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Quantum Theory of Atoms and Molecules

Cited by 66 publications

References 56 publications

Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states

Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states

Scaled Density Functional Theory Correlation Functionals

Perturbative configuration interaction using localized orbitals in the INDO hypothesis. I. Theory and applications to energetical problems

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