LAUSR

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 ( V i a and V j a ). Furthermore, the second-order differentiation can be evaluated accurately from the energy gaps, ( V i a - V j a ) , 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 3 3 Π and 4 3 Π states of NaK, between the 1 3 B 3 u and 2 3 B 3 u states of N2, and between the 2 3 A ″ and 3 3 A ″ states of methylene (CH2).