Quasi-diabatic states and dynamical couplings from ab initio CI calculations: a new proposal

A new proposal for determining CI quasi-diabatic states, potential energy surfaces and electronic couplings is introduced using the quasi-degenerate perturbation theory (QDPT); zeroth-order CI adiabatic states, resulting from a partial diagonalisation, are submitted to a unitary transformation in order to yield quasi-diabatic states presenting the maximum overlap with an external set of independent diabatic reference functions. An effective Hamiltonian in the quasi-diabatic representation is then obtained by applying the QDPT in the model space spanned by the quasi-diabatic states. This algorithm is exemplified by the HeNe 2+1 Sigma + states for which the dynamical couplings are computed using finite difference techniques combined with first-order transition density matrices.