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Molecular vertical excitation energies studied with first-order RASSCF (RAS[1,1]): balancing covalent and ionic excited states

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Title: Molecular vertical excitation energies studied with first-order RASSCF (RAS[1,1]): balancing covalent and ionic excited states
Authors: Tran, T
Segarra-Martí, J
Bearpark, M
Robb, MA
Item Type: Journal Article
Abstract: RASSCF calculations of vertical excitation energies were carried out on a benchmark set of 19 organic molecules studied by Thiel and co-workers [ J. Chem. Phys. 2008, 128, 134110]. The best results, in comparison with the MS-CASPT2 results of Thiel, were obtained using a RASSCF space that contains at most one hole and one particle in the RAS1 and RAS3 spaces, respectively, which we denote as RAS[1,1]. This subset of configurations recovers mainly the effect of polarization and semi-internal electronic correlation that is only included in CASSCF in an averaged way. Adding all-external correlation by allowing double excitations from RAS1 and RAS2 into RAS3 did not improve the results, and indeed, they were slightly worse. The accuracy of the first-order RASSCF computations is demonstrated to be a function of whether the state of interest can be classified as covalent or ionic in the space of configurations built from orbitals localized onto atomic sites. For covalent states, polarization and semi-internal correlation effects are negligible (RAS[1,1]), while for ionic states, these effects are large (because of inherent diffusiveness of these states compared to the covalent states) and, thus, an acceptable agreement with MS-CASPT2 can be obtained using first-order RASSCF with the extra basis set involving 3p orbitals in most cases. However, for those ionic states that are quasi-degenerate with a Rydberg state or for nonlocal nπ* states, there remains a significant error resulting from all external correlation effects.
Issue Date: 27-Jun-2019
Date of Acceptance: 31-May-2019
URI: http://hdl.handle.net/10044/1/70736
DOI: 10.1021/acs.jpca.9b03715
ISSN: 1089-5639
Publisher: American Chemical Society
Start Page: 5223
End Page: 5230
Journal / Book Title: Journal of Physical Chemistry A
Volume: 123
Issue: 25
Copyright Statement: © 2019 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY)License, which permits unrestricted use, distribution and reproduction in any medium,provided the author and source are cited.
Sponsor/Funder: Commission of the European Communities
Funder's Grant Number: 747662
Keywords: Science & Technology
Physical Sciences
Chemistry, Physical
Physics, Atomic, Molecular & Chemical
0307 Theoretical and Computational Chemistry
0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics
0306 Physical Chemistry (incl. Structural)
Publication Status: Published
Online Publication Date: 2019-05-31
Appears in Collections:Chemistry
Faculty of Natural Sciences