Accurate first-derivative nonadiabatic couplings for the H-3 system

A conical intersection exists between the ground (1 (2) A') and the first-e xcited (2 (2)A') electronic potential energy surfaces (PESs) of the H-3 sys tem for C-3 upsilon geometries. This intersection induces a geometric phase effect, an important factor in accurate quantum mechanical reactive scatte ring calculations, which at low energies can be performed using the ground PES only, together with appropriate nuclear motion boundary conditions. At higher energies, however, such calculations require the inclusion of both t he 1 (2)A' and 2 (2)A' electronic PESs and the corresponding nuclear deriva tive couplings. Here we present ab initio first-derivative couplings for th ese states obtained by analytic gradient techniques and a fit to these resu lts. We also present a fit to the corresponding 1 (2)A' and 2 (2)A' adiabat ic electronic PESs, obtained from the ab initio electronic energies. The fi rst-derivative couplings are compared with their approximate analytical cou nterparts obtained by Varandas [J. Chem. Phys. 86, 6258 (1987)] using the d ouble many-body expansion method. As expected, the latter are accurate clos e to conical intersection configurations but not elsewhere. We also present the contour integrals of the ab initio couplings along closed loops around the above-mentioned conical intersection, which contain information about possible interactions between the 2 (2)A' and 3 (2)A' states. (C) 2001 Amer ican Institute of Physics.