Theory of Atomic Structure Including Electron Correlation. I. Three Kinds of Correlation in Ground and Excited Configurations

Öksüz, İskender; Si̇nanoğlu, Oktay

doi:10.1103/physrev.181.42

Cited by 223 publications

(35 citation statements)

References 18 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 energy obtained at the MRCI level includes some additional electron correlation energy, in com parison with the CASSCF energy, (9) This energy is named the external or dynamical corre lation energy [69,70]; it arises from the non specific instantaneous correlation motion of electrons and was introduced by Sinano lu [71,72].…”

Section: Methodsmentioning

confidence: 99%

“…Sinano lu and coworkers [71][72][73] showed that the correlation energy can be roughly divided into two types: dynamical correlations, which are transferable from system to system, and non dynamical correla tions, which are not. Usually non dynamical correla tions are responsible for a correct description of the dissociation products and in some papers is named left right correlation [74].…”

Section: Methodsmentioning

confidence: 99%

Multi-reference Ab initio calculations of 3d transition-metal dimers: Sc2

Kaplan

Miranda

2014

Russ. J. Phys. Chem.

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The problem of calculation of the electronic structure of transition metal clusters (even dimers) still presents a challenge for computational chemistry. The reason is that the expansion of the ground state wave function on electronic configurations does not contain a principal configuration and a large number of reference configurations must be treated equally. Thus, the multi reference (MR) approaches are, in general, mandatory. A short description of the MRCI methods is presented. The calculation of Sc 2 at the MRCISD(+Q)/cc pV5Z level, showed that the its ground state corresponds to a quintet, , in agreement with experiment and previous precise calculations. The triplet state is located about 1.1 kcal/mol above.We calculated the ground state, X , of the Sc 2 dimer by the MRCISD(+Q) method at the complete basis set (CBS) limit. This is the first MRCISD(+Q) calculation of 3d transition metal clusters at the CBS limit. From the Mulliken population analysis and comparison with atomic energies follows that in the ground state Sc 2 dissociates on one Sc in the ground state and the other in the second excited quartet state, 4 F u . The spec troscopic parameters of the ground potential curve, obtained by the Dunham analysis at the valence MRCISD(+Q)/CBS level, are: R e = 5.20 bohr, D e = 50.37 kcal/mol, and e = 234.5 cm -1 . The obtained value for the harmonic frequency agrees very well with the experimental one, e = 239.9 cm -1 . From the indistin guishability of identical atoms in quantum mechanics follows that in spite of an asymmetric dissociation limit for Sc 2 , both Sc atoms have the same electron population and it is incorrect to consider them as different. Therefore, the accepted classification of the dimer states should be revised, since some of them are forbidden by the Pauli Exclusion Principle.Keywords: 3d transition metal dimers, Sc 2 , multi reference configuration interaction (MRCI), complete basis set limit.

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“…It is now broadly recognized that the many‐electron problem is best analysed and tackled computationally if a space of strongly mixing configurations is used to construct a zero order wave function with self‐consistently optimized orbitals for the state of interest. This is the notion of the Fermi‐Sea (FS) zero order reference wave function 1, 10, 12, 13, which is the improvement and generalization of Sinanoğlu's choice of the “Hartree–Fock Sea” that was defined in the 1960s for first‐row atoms by hydrogenic degeneracy and led to the introduction and application of the concepts of nondynamical and dynamical correlation effects 28–30.…”

Section: Comment On Two Concepts and Methods In The Theory And Commentioning

confidence: 99%

On calculations of correlated wave functions with heavy configurational mixing

Nicolaides¹

2004

Int J of Quantum Chemistry

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ABSTRACT:We discuss aspects of the theory and computation of wave functions and energies of discrete states of polyelectronic atoms that are represented in zero order by configurations with holes in subshells below the valence subshell. Both in zero order and in the remaining correlation components, such wave functions have particularities stemming from the state-specific self-consistent field and the heavy configurational mixing associated with near-degeneracies and hole-filling correlations. By referring to a variety of examples from small-and large-scale calculations, it is noted that appropriate penetration into the many-body problem can provide, in an economic and physically transparent way, reliable interpretations and semi-and fully quantitative understanding of issues related to states with inner holes and to cases of near-degeneracies that result in strongly correlated wave functions. Whenever hole-filling correlations are allowed, multiple correlations (i.e., beyond single-and double-orbital substitutions in the single reference configuration) acquire increased importance relative to that in ordinary electronic structures. This is demonstrated via large-scale multiconfigurational HartreeFock (MCHF) plus configuration interaction (CI) calculations on the Cl KL3s3p 6 2 S discrete state, which is the lowest of its symmetry. The calculations incorporated correlations up to selected sextuple orbital excitations from the M shell. MCHF plus CI calculations at the level of quadruple orbital substitutions were also carried out for the Cl KL3s 2 3p 5 2 P o ground state and the excitation energy at this level of calculation was found to be 85,364 cm Ϫ1 , in excellent agreement with the experimental value of the finestructure-weighted average, 85,385 cm Ϫ1 (10.59 eV). Within the approximations of the calculation, the hole-filling triple and quadruple orbital correlations, which, of course, are absent from the 2 P o state, contribute about 1 eV, which is significant.

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Theory of Atomic Structure Including Electron Correlation. I. Three Kinds of Correlation in Ground and Excited Configurations

Cited by 223 publications

References 18 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

Multi-reference Ab initio calculations of 3d transition-metal dimers: Sc2

On calculations of correlated wave functions with heavy configurational mixing

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