Beyond Born-Oppenheimer treatment for the construction of triple-sheeted accurate diabatic Hamiltonian matrix of F+H₂ system

Abstract

We intend to study the non-adiabatic interactions among the three lowest adiabatic states (1²A', 1²A'', and 2²A') of F+H₂ triatomic reactive system in hyperspherical coordinates for a fixed hyperradius at ρ = 7.5 bohr as functions of hyperangles, Θ (0° ≤ Θ ≤ 90°) and ɸ (0° ≤ ɸ ≤ 360°). The adiabatic potential energy surfaces are calculated using MRCI level of methodology whereas the non-adiabatic coupling terms between those states are calculated from the analytic gradient methods implemented in MOLPRO quantum chemistry package. The ground (1²A') and the first excited (1²A'') states exhibit conical intersection (CI) and seam of CI along C_2v geometries, whereas the first (1²A'') and the second (2²A') excited states undergo Renner-Teller coupling at linear geometries. We carry out adiabatic-to-diabatic transformation (ADT) by solving ADT equations to obtain ADT angles for constructing single-valued, continuous and symmetric 3 × 3 diabatic potential energy matrix so that subsequent accurate scattering calculations can be performed.