Inclusion of second-order correlation effects for the ground and singly-excited states suitable for the study of conical intersections: The CIS(2) model

A new electronic structure model is developed in which the energies of both the ground and singly-excited states are eigenvalues of a dressed symmetric configuration interaction (CI) matrix in the space of the reference and singly-excited determinants. The effects of double and triple substitutions are approximately included into the CI matrix elements in the spirit of quasidegenerate second-order perturbation theory. The model correctly describes conical intersections between the ground and singly-excited states and appears to be the simplest single-reference correlated treatment for this class of problems. Test calculations on organic molecules are presented.