Non-adiabatic effects play a central role in the spectroscopy and dynamics of open-shell and electronically excited molecules, but the vibronic coupling implicit in such systems poses both computational and conceptual challenges. Although a variety of sophisticated time- and frequency-domain techniques exist that accurately treat non-adiabatic molecules, such methods are often expensive and scale steeply with molecular size. This talk will describe some perturbative approaches to the vibronic problem, with a particular focus on the low-energy structure of Jahn-Teller and pseudo-Jahn-Teller molecules within a quasi-diabatic framework. We will discuss the minimal zeroth-order models that accurately capture the essential correlation between electronic and vibrational degrees of freedom and demonstrate that perturbative corrections based on them can provide accurate predictions even for strongly coupled systems. Some interesting phenomena not usually encountered in pure vibrational problems, such as mean-field symmetry breaking, appear along the way. We hope this approach can ultimately be developed into an efficient and accurate method for predicting spin-rovibronic spectroscopic Hamiltonians of small and medium-sized molecules.

Vibronic perturbation theory for Jahn-Teller and pseudo-Jahn-Teller molecules

ACS Spring 2022

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