A novel perturbation-based CAS-SCF algorithm: application to the direct calculation of localized orbitals

A complete active space-self-consistent-field (CAS-SCF) algorithm based on molecular orbitals that conserve their physical nature during the iterative process is proposed. The algorithm consists of an iterative procedure based on the imposition of the generalized Brillouin theorem to a complete active space-configuration interaction wave function. At convergence, the wave function is identical to the corresponding one obtained using canonical CAS-SCF orbitals, provided the nature of the active space is the same. If localized guess orbitals are used, the locality property is conserved by the final orbitals. Test calculations illustrate the interest of the proposed approach, that permits to control the nature of the active space.