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

A Kohn-Sham-type computational scheme capable of treating systems with stro ng nondynamic correlation is presented. The scheme, dubbed the spin-restric ted, ensemble-referenced Kohn-Sham (REKS) method, is based on the represent ation of the density and energy for a strongly correlated system as weighte d 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 impor tant situations: (1) avoided crossing of potential energy surfaces, (2) bon d-breaking processes, and (3) electronic structure of diradicals. The resul ts 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 availa ble density functional approaches.