Local orbitals for quasi-degenerate systems

A CAS-SCF algorithm based on molecular orbitals that conserve their physical character during the iterative process has been developed. The method is based on the iterative partial diagonalization of the one-body density matrix, obtained from a Configuration Interaction restricted to the space of single excitations from the CAS. When localized guess orbitals are used, the locality property is conserved for the final orbitals. This localization technique is particularly suitable for the treatment of quasi-degenerated systems, since it can be applied to those cases that cannot be correctly described at SCF level. For large systems, the use of localized active orbitals leads to a huge reduction of the computational effort, and permits MR-CI treatments that would be out of the possibilities of the standard delocalized approaches.