A comparative study of methods for describing non-adiabatic coupling: diabatic representation of the 1Σ+/1Π HOH and HHO conical intersections

Abstract

A number of established techniques for obtaining diabatic electronic states in small molecules are critically compared for the example of the [Xtilde] and [Btilde] states in the water molecule, which contribute to the two lowest-energy conical intersections. Integration of the coupling matrix elements and analysis of configuration mixing coefficients both produce reliable diabatic states globally. Methods relying on diagonalization of dipole moment and angular momentum operators are shown to fail in large regions of coordinate space. However, the use of transition angular momentum matrix elements involving the A state, which is degenerate with [Btilde] at the conical intersections, is successful globally, provided that an appropriate choice of coordinates is made. Long range damping of non-adiabatic coupling to give correct asymptotic mixing angles also is investigated.