Ab Initio Calculations and Modeling of Three-Dimensional Adiabatic and Diabatic Potential Energy Surfaces of Br(2P)···H2(1Σ+) Pre-Reactive Complex

Abstract

The three lowest diabatic potential energy surfaces for the Br(2P) + H2 van der Waals complex are derived from accurate ab initio calculations of the T-shaped (C2v) and collinear geometries (C∞v), at the coupled cluster with single, double, and noniterative triple excitations (CCSD(T)) level of theory with a large basis set. For the intermediate geometries, the angular dependence is modeled by a simple Legendre-polynomial interpolation. The nonadiabatic coupling (off-diagonal derivative) matrix element and the fourth, off-diagonal, diabatic surface are determined by separate complete active space self-consisted field (CASSCF) calculations with a medium-sized basis set. Three adiabatic potential energy surfaces (PESs) are also obtained, by diagonalizing the total Hamiltonian in the diabatic basis. Both the nonrelativistic and relativistic (including spin−orbit coupling) PESs are evaluated. The dependence of the PESs on the H2 stretching coordinate is also incorporated and analyzed.