Molecular Vertical Excitation Energies Studied with First-Order RASSCF (RAS[1,1]): Balancing Covalent and Ionic Excited States.

Thierry Tran,Michael J Bearpark,Michael A Robb,Javier Segarra-Martí

doi:10.1021/acs.jpca.9b03715

Thierry Tran, Michael J Bearpark + Show 2 more

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https://doi.org/10.1021/acs.jpca.9b03715

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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, 13411018397056]. 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.

Highlights

The theoretical study of excited state reactivity using nonadiabatic molecular dynamics represents a challenge for electronic structure methods because they must account for a changing importance of open versus closed shell configurations and covalent versus ionic character in the wave function.1 This balance involves electronic correlation effects
The main aim of this work is to demonstrate the accuracy of the restricted active space self-consistent field (RASSCF) approach for the computation of valence singlet excited states and, the potential suitability of the method for investigating potential energy surfaces relevant for photochemistry using on-the-fly ab initio nonadiabatic dynamics that require analytical derivatives
This work will be discussed after we present our approach, which is focused on the improvement of complete active space selfconsistent field (CASSCF) results using a RASSCF approach

Summary

Introduction

The theoretical study of excited state reactivity using nonadiabatic molecular dynamics represents a challenge for electronic structure methods because they must account for a changing importance of open versus closed shell configurations and covalent versus ionic character in the wave function. This balance involves electronic correlation effects. The theoretical study of excited state reactivity using nonadiabatic molecular dynamics represents a challenge for electronic structure methods because they must account for a changing importance of open versus closed shell configurations and covalent versus ionic character in the wave function.. The theoretical study of excited state reactivity using nonadiabatic molecular dynamics represents a challenge for electronic structure methods because they must account for a changing importance of open versus closed shell configurations and covalent versus ionic character in the wave function.1 One needs a balanced description of excited states (i.e., comparable excitation energies and ordering of states) of different character with the same relative accuracy. The challenge is to include electron correlation effects via theoretical methods that are sufficiently efficient to perform the very many energy evaluations required in nonadiabatic dynamics. The theoretical method used needs to permit the computation of first and second derivatives analytically (as opposed to finite difference computation) as well as the necessary derivative couplings

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