Communication: Conical intersections using constrained density functional theory-configuration interaction.

The constrained density functional theory-configuration interaction (CDFT-CI) method has previously been used to calculate ground-state energies and barrier heights. In this work, it is examined for use in computing electronic excited states, for the challenging case of conical intersections. Conical intersections are a prevalent feature of excited electronic surfaces, but conventional time-dependent density functional theory calculations are found to be entirely unsatisfactory at describing them, for two small systems. CDFT-CI calculations on those systems are found to be in qualitative agreement with reference CAS surfaces. These results suggest that with a suitable definition of atomic populations and a careful choice of constrained states, CDFT-CI could be the basis for a seamless description of electronic degeneracy.