On the consequences of nonremovable derivative couplings. I. The geometric phase and quasidiabatic states: A numerical study

Conical intersections complicate the computational treatment of nuclear dynamics in the adiabatic state basis through the geometric phase effect and singularities in the derivative couplings. The diabatic representation seeks to eliminate these difficulties. However, the adiabatic to diabatic state transformation is necessarily approximate in a polyatomic molecule since the derivative couplings cannot be rigorously removed. This point is rarely considered when constructing approximate diabatic states. The nonremovable part of the derivative couplings is investigated by considering the integral of the derivative coupling along closed loops in the vicinity of the 1 ²A'-2 ²A' seam of conical intersections in H₃.