On the nonadiabatic collisional quenching of OH(A) by H2: a four coupled quasi-diabatic state description

A four-state diabatic potential energy matrix (DPEM),H-d, for the description of the nonadiabatic quenching of OH(A(2)sigma(+)) by collisions with H(2)is reported. The DPEM is constructed as a fit to adiabatic energies, energy gradients, and derivative couplings obtained exclusively from multireference configuration interaction wave functions. A four-adiabatic-electronic-state representation is used in order to describe all energetically accessible regions of the nuclear coordinate space. Partial permutation-inversion symmetry is incorporated into the representation. The fit is based on electronic structure data at 42 882 points, described by over 1.6 million least squares equations with a root mean square (mean unsigned) error of 178(83) cm(-1). Comparison ofab initioandH(d)determined minima, saddle points, and energy minimized points onC(2v),C-s,C-infinity v, andC(1)(noncoplanar) portions of two conical intersection seams are used to establish the accuracy of theH(d).