Practical and reliable approximation of nonadiabatic coupling terms between triplet electronic states using only adiabatic potential energies

Abstract

The nonadiabatic coupling terms (NACTs, Fij’s) between triplet electronic states can be well approximated with a very simple equation suggested recently [J. Chem. Phys. 146, 064107 (2017)], using only adiabatic potential energies (Via and Vja). Furthermore, the second-order differentiation can be evaluated accurately from the energy gaps, (Via-Vja), at just three consecutive geometries, even at a point exhibiting severe singularity in the behaviour of NACTs. The performance of this approximation method was investigated for the couplings between the 33Π and 43Π states of NaK, between the 13B3u and 23B3u states of N2, and between the 23A″ and 33A″ states of methylene (CH2).