Diradicaloids: Description by the spin-restricted, ensemble-referenced Kohn-Sham density functional method

A Kohn-Sham-type computational scheme capable of treating systems with strong nondynamic correlation is presented. The scheme, dubbed the spin-restricted, ensemble-referenced Kohn-Sham (REKS) method, is based on the representation of the density and energy for a strongly correlated system as weighted sums of densities and energies of several Kohn-Sham (KS) determinants. An optimal set of orthonormal KS orbitals and occupation numbers is obtained by minimizing the ground-state energy as a function of the density. Results of REKS calculations are reported and cover the following chemically important situations: (1) avoided crossing of potential energy surfaces, (2) bond-breaking processes, and (3) electronic structure of diradicals. The results of REKS calculations are compared with the available Kohn-Sham solutions for cases in which the exact density is known, as well as with results of conventional multireference ab initio methods and with the currently available density functional approaches.