A spin-restricted ensemble-referenced Kohn-Sham method and its application to diradicaloid situations

The energy and density of situations with strong non-dynamic correlation are formulated as weighted sums (ensembles) of energies and densities of symmetry-adapted reference KS determinants. A computational scheme termed the spin-restricted ensemble-referenced Kohn-Sham (REKS) method is devised for these cases. An optimal set of orthonormal one-electron orbitals and their optimal occupation numbers are obtained from minimization of the ground state energy with respect to the density. The REKS method is applied to several model problems, rotation in C₂H₄, dissociation of H₂, and the singlet-triplet energy gaps in substituted trimethylene diradicals.