Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states

Abstract

(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.

Ultrafast Photoinduced Dynamics of 1,3-Cyclohexadiene Using XMS-CASPT2 Surface Hopping

“…12,[19][20][21]23 Significantly increasing the size of (restricted) active space in a consistent manner has been shown to allow for a qualitatively correct state ordering for the butadiene-like molecules. 43 However, quantitatively correct state energies are harder to obtain, while RASSCF with very large active space can be expected to be rather expensive and hard to deal with in dynamics calculations. Martínez et al and Lei et al used clipped active spaces that included only a single (π * -) virtual orbital (CAS(6,4) 26,44,45 and CAS(14,8) 29 correspondingly), which, while giving the qualitatively right order of states in the FC region, probably reach this effect through error compensation, and are thus rather dangerous to be used in full dimensional on-the-fly dynamic calculations, where far-lying regions of the configuration space may be sampled.…”

Theoretical study of cyclohexadiene/hexatriene photochemical interconversion using spin-Flip time-Dependent density functional theory

“…This fact has limited the CASSCF method to describe this reaction process approximately after the crossing between S 2 and S 1 [8,9]. Santolini et al [27] have obtained the correct order of the states in the FC region for the butadiene-like molecules by significantly increasing the size of the restricted active space (RAS). Although this demonstrates that the correct order of the states can be qualitatively achieved, the process of obtaining them for more complicated systems can be hard, and demands an increase in computational complexity [22,27].…”

“…Santolini et al [27] have obtained the correct order of the states in the FC region for the butadiene-like molecules by significantly increasing the size of the restricted active space (RAS). Although this demonstrates that the correct order of the states can be qualitatively achieved, the process of obtaining them for more complicated systems can be hard, and demands an increase in computational complexity [22,27]. The second-order perturbation theory for CASSCF method (CASPT2) is another widely used multireference method that has shown very good results [5,18,22].…”

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

“…Santolini et al 88 used CASSCF and RASSCF to study the photochemical reactions of cis-butadiene and cyclopentadiene. It was found that small CASSCF active spaces result in a poor balance of covalent and ionic valence bond (VB) structures, and an extended active space is therefore required for accurate potential energy surfaces.…”