The determination of radial non-adiabatic coupling: HeNe2+as a case study

The matrix elements of the operator d/dR between the first three 1 Sigma + singlets of HeNe 2+ have been calculated by the rigorous finite differences technique and by Levy's projection method, based on the determination of the effective Hamiltonian in a quasi-diabatic basis. A numerical diabatisation procedure based on the knowledge of the d/dR matrix elements has also been applied. The results of the two methods for determining non-adiabatic couplings and quasidiabatic states have been compared. The nature of the avoided crossing between the A and B 1 Sigma + states of HeNe 2+ is discussed.