Investigation of Herzberg–Teller Franck–Condon approaches and classical simulations to include effects due to vibronic coupling in circular dichroism spectra: The case of dimethyloxirane continued

Abstract

AbstractA recent study has shown that the inclusion of vibrational effects may be vital in first‐principles simulations of circular dichroism (CD) spectra. In the present work, we show that effects from vibronic coupling or, alternatively, Herzberg–Teller corrections, can have additional and perhaps unusually strong effects, due to delicate cancellations between positive and negative contributions in a CD spectrum. As full vibronic coupling simulations tend to be too demanding computationally for the sizeable molecules considered in CD spectroscopy, it is pertinent to investigate suitable approximations. In the first part of this study, we examine various Herzberg–Teller Franck–Condon (HT–FC) approaches that include the geometrical dependence of the rotatory strength for a 3 × 3 linear vibronic model, for which exact numerical results are easily obtained. Somewhat disappointingly, none of the HT–FC approaches investigated here yielded sufficiently satisfactory results. In a subsequent part of this study, we show that purely classical simulations, based on a vibronic model, can provide reliable spectra and appear to be well suited to address the dependence of the rotatory strength on nuclear geometry. From the classical simulations we extract average rotatory strengths for each electronic state, which are then used in subsequent gradient FC calculations to incorporate vibrational fine structure in the CD spectrum. This mixed classical/gradient FC approach is applied to the methyloxirane CD spectrum, using the statistical averaging of (model) orbital potentials–time‐dependent density functional theory (SAOP–TDDFT) vibronic model presented recently and is shown to improve substantially on the original simulation, corroborating the potential importance of vibronic coupling effects in simulations of CD spectra. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006