Beyond Born–Oppenheimer Constructed Diabatic Potential Energy Surfaces for HeH<sub>2</sub><sup>+</sup>
Koushik Naskar, Satyam Ravi, Satrajit Adhikari, Michael Baer, N. Sathyamurthy
Abstract
First-principles based beyond Born–Oppenheimer theory has been employed to construct multistate global Potential-Energy Surfaces (PESs) for the HeH 2 + system by explicitly incorporating the Nonadiabatic Coupling Terms (NACTs). Adiabatic PESs and NACTs for the lowest four electronic states (1 2 A′, 2 2 A′, 3 2 A′ and 4 2 A′) are evaluated as functions of hyperangles for a grid of fixed values of the hyperradius in hyperspherical coordinates. Conical intersection between different states are validated by integrating the NACTs along appropriately chosen contours. Subsequently, adiabatic-to-diabatic (ADT) transformation angles are determined by solving the ADT equations to construct the diabatic potential matrix for the HeH 2 + system which are smooth, single-valued, continuous, and symmetric and are suitable for performing accurate scattering calculations for the titled system.