Imperial College London
Alternate

Professor Mike Robb, FRS

Faculty of Natural Sciences, Department of Chemistry

Chair in Chemistry
 
 
 
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Contact

 

+44 (0)20 7594 5757mike.robb Website

 
 
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Location

 

301cMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Citation

BibTex format

@article{Tran:2019:10.1021/acs.jpca.9b03715,
author = {Tran, T and Segarra-Martí, J and Bearpark, M and Robb, MA},
doi = {10.1021/acs.jpca.9b03715},
journal = {Journal of Physical Chemistry A},
pages = {5223--5230},
title = {Molecular vertical excitation energies studied with first-order RASSCF (RAS[1,1]): balancing covalent and ionic excited states},
url = {http://dx.doi.org/10.1021/acs.jpca.9b03715},
volume = {123},
year = {2019}
}
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RIS format (EndNote, RefMan)

TY  - JOUR
AB  - 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.
AU  - Tran,T
AU  - Segarra-Martí,J
AU  - Bearpark,M
AU  - Robb,MA
DO  - 10.1021/acs.jpca.9b03715
EP  - 5230
PY  - 2019///
SN  - 1089-5639
SP  - 5223
TI  - Molecular vertical excitation energies studied with first-order RASSCF (RAS[1,1]): balancing covalent and ionic excited states
T2  - Journal of Physical Chemistry A
UR  - http://dx.doi.org/10.1021/acs.jpca.9b03715
UR  - http://hdl.handle.net/10044/1/70736
VL  - 123
ER  -
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