Five electronic state beyond Born–Oppenheimer equations and their applications to nitrate and benzene radical cation

Abstract

We present explicit form of Adiabatic to Diabatic Transformation (ADT) equations and expressions of non-adiabatic coupling terms (NACTs) for a coupled five-state electronic manifold in terms of ADT angles between electronic wave functions. ADT matrices eliminate the numerical instability arising from singularity of NACTs and transform the adiabatic Schrödinger equation to its diabatic form. Two real molecular systems NO₃ and C₆H₆+ (Bz⁺) are selectively chosen for the demonstration of workability of those equations. We examine the NACTs among the lowest five electronic states of the NO₃ radical [X̂²A'₂(1²B₂), ²E"(1²A₂ and 1²B₁), B̂²E'(1²A₁ and 2²B₂)], in which all types of non-adiabatic interactions, that is, Jahn–Teller (JT) interactions, Pseudo Jahn–Teller (PJT) interactions, and accidental conical intersections (CIs) are present. On the other hand, lowest five electronic states of Bz⁺ [X̂²E_1g(1²B_3g and 1²B_2g), B̂²E_2g(1²A_g1²B_1g) and Ĉ²A_2u(1²B_1u)] depict similar kind of complex feature of non-adiabatic effects. For NO₃ radical, the two components of degenerate in-plane asymmetric stretching mode are taken as a plane of nuclear configuration space (CS), whereas in case of Bz⁺, two pairs are chosen: One is the pair of components of degenerate in-plane asymmetric stretching mode, and the other one is constituted with one of the components each from out-of-plane degenerate bend and in-plane degenerate asymmetric stretching modes. We calculate ab initio adiabatic potential energy surfaces (PESs) and NACTs among the lowest five electronic states at the CASSCF level using MOLPRO quantum chemistry package. Subsequently, the ADT is performed using those newly developed equations to validate the positions of the CIs, evaluate the ADT angles and construct smooth, symmetric, and continuous diabatic PESs for both the molecular systems.