Analytic evaluation of the nonadiabatic coupling vector between excited states using equation-of-motion coupled-cluster theory.

Theory and implementation for evaluation of the nonadiabatic coupling vector between excited electronic states described by equation-of-motion excitation energy coupled-cluster singles and doubles (EOMEE-CCSD) method is presented. Problems arising from the non-Hermitian nature of the theory are discussed in detail. The performance of the new approach is demonstrated by the nice agreement of the nonadiabatic coupling curves for LiH obtained at the EOMEE-CCSD and MR-CISD levels. Using the tools developed we also present a computational procedure to evaluate the interstate coupling constants used in vibronic coupling theories. As an application of this part of the implementation we present simulation of the electronic absorption spectrum of the pyrazine molecule within the linear vibronic coupling model.