Localized orbitals in a Multi-Reference context

Variational and Perturbative CAS-SCF-type algorithms based on molecular orbitals that preserve their physical nature during the iterative process are discussed. The methods are based on the iterative diagonalization of the one-body reduced density matrix. If localized guess orbitals are used, the locality property is kept by the final orbitals. The formalism can be used to reduce the number of active orbitals in CAS-SCF calculations on large systems, and in general to have a better control on the physical nature of the active space. The reduction from a complete to selected reference space is also possible in the case of Configuration-Interaction calculations.