Choosing RASSCF orbital active spaces for multiple electronic states

Krausbeck, Florian; Mendive-Tapia, David; Thom, Alex J. W.; Bearpark, Michael J.

doi:10.1016/j.comptc.2014.03.030

Cited by 25 publications

(22 citation statements)

References 19 publications

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“…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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“…In larger molecules, one may average the density matrices obtained from several SCF calculations (ground state and a few excited states) as a means to occupy more of the valence orbitals. 28 A systematic procedure for obtaining those few empty valence orbitals involved in "strong correlation" is to 29,30 (known as the UNO-CAS method).…”

Section: Introductionmentioning

confidence: 99%

Valence Virtual Orbitals: An Unambiguous ab Initio Quantification of the LUMO Concept

2015

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Many chemical concepts hinge on the notion of an orbital called the lowest unoccupied molecular orbital, or LUMO. This hypothetical orbital and the much more concrete highest occupied molecular orbital (HOMO) constitute the two "frontier orbitals", which rationalize a great deal of chemistry. A viable LUMO candidate should have a sensible energy value, a realistic shape with amplitude on those atoms where electron attachment or reduction or excitation processes occur, and often an antibonding correspondence to one of the highest occupied MOs. Unfortunately, today's quantum chemistry calculations do not yield useful empty orbitals. Instead, the empty canonical orbitals form a large sea of orbitals, where the interesting valence antibonds are scrambled with the basis set's polarization and diffuse augmentations. The LUMO is thus lost within a continuum associated with a detached electron, as well as many Rydberg excited states. A suitable alternative to the canonical orbitals is proposed, namely, the valence virtual orbitals. VVOs are found by a simple algorithm based on singular value decomposition, which allows for the extraction of all valence-like orbitals from the large empty canonical orbital space. VVOs are found to be nearly independent of the working basis set. ABSTRACT: Many chemical concepts hinge on the notion of an orbital called the lowest unoccupied molecular orbital, or LUMO. This hypothetical orbital and the much more concrete highest occupied molecular orbital (HOMO) constitute the two "frontier orbitals", which rationalize a great deal of chemistry. A viable LUMO candidate should have a sensible energy value, a realistic shape with amplitude on those atoms where electron attachment or reduction or excitation processes occur, and often an antibonding correspondence to one of the highest occupied MOs. Unfortunately, today's quantum chemistry calculations do not yield useful empty orbitals. Instead, the empty canonical orbitals form a large sea of orbitals, where the interesting valence antibonds are scrambled with the basis set's polarization and diffuse augmentations. The LUMO is thus lost within a continuum associated with a detached electron, as well as many Rydberg excited states. A suitable alternative to the canonical orbitals is proposed, namely, the valence virtual orbitals. VVOs are found by a simple algorithm based on singular value decomposition, which allows for the extraction of all valence-like orbitals from the large empty canonical orbital space. VVOs are found to be nearly independent of the working basis set. The utility of VVOs is demonstrated for construction of qualitative MO diagrams, for prediction of valence excited states, and as starting orbitals for more sophisticated calculations. This suggests that VVOs are a suitable realization of the LUMO, LUMO + 1, ... concept. VVO generation requires no expert knowledge, as the number of VVOs sought is found by counting s-block atoms as having only a valence s orbital, transition metals as having valence s and d, and main ...

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“…In the present work we have explored the S 2 potential energy surface (PES) of benzene by employing the Restricted Active Space Self‐Consistent Field (RASSCF) method which is a multireference wave function method describing adequately the mixing of electronic configurations and degeneracy effects and depicting very well the topology of the PESs. According to this method we choose an active space of the most important orbitals and then the initial space is partitioned into three subspaces called RAS1, RAS2 and RAS3.…”

Section: Figurementioning

confidence: 99%

On the Problem of Multiple Minima on the S2 Excited Potential Energy Surface of Benzene: A Restricted Active Space Self Consistent Field Study

2020

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Application of the Restricted Active Space Self‐Consistent Field Theory on the S2 excited state potential energy surface of benzene shows the existence of three minima. Of these, the first one is planar with a D6h molecular point group, while the other two have a boat type structure with an approximate C2v symmetry. All three minima have a biradicaloid structure and constitute potential candidates for new photochemical pathways on the S2 surface. A common S2/S1 Conical Intersection, which is accessible from all three S2 minima has been found which decays to the S1 potential energy surface (PES) giving benzene S1 minimum as the only photoproduct.

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Choosing RASSCF orbital active spaces for multiple electronic states

Cited by 25 publications

References 19 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

Valence Virtual Orbitals: An Unambiguous ab Initio Quantification of the LUMO Concept

On the Problem of Multiple Minima on the S2 Excited Potential Energy Surface of Benzene: A Restricted Active Space Self Consistent Field Study

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