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

The nonadiabatic coupling terms (NACTs, F-ij'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)Pi and 4(3)Pi states of NaK, between the 1(3)B(3u) and 2(3)B(3u) states of N-2, and between the 2(3)A '' and 3(3)A '' states of methylene (CH2). (C) 2018 Elsevier B.V. All rights reserved.