Beyond Born-Oppenheimer Constructed Diabatic Potential Energy Surfaces for H₃: Adiabatic and Diabatic Reaction...

We construct a multistate global diabatic Potential Energy Surface (PES) matrix for the H₃ system by employing first-principles-based Beyond Born-Oppenheimer theory, where ab initio-calculated adiabatic PESs and Nonadiabatic Coupling Terms (NACTs) for the lowest three electronic states (1²A', 2²A', 3²A') are incorporated. These ab initio calculations are carried out as functions of hyperangles, θ and ϕ, for a grid of fixed values of the hyperradius (ρ) by engaging an accurate methodology, namely, CASSCF followed by the MRCI level of approaches. Conical intersections between different electronic states are located by integrating the NACTs along appropriately chosen contours using the quantization properties of singular functions. Finally, adiabatic-to-diabatic (ADT) transformation angles are obtained by solving the ADT equations to construct the diabatic potential matrix for the H₃ system, which is smooth, single-valued, continuous, symmetric and suitable for performing accurate scattering calculations for the titled system.