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A CAS-SCF algorithm based on molecular orbitals that conserve their physical character during the iterative process has recently been developed in our group in Toulouse. 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 at convergence.
This localization technique is particularly suitable for the treatment of quasi-degenerate systems, since it can be applied to those cases that cannot be correctly described at the SCF level. Once local orbitals of CAS-SCF quality have been obtained, it is possible to perform a Multi-Reference Configuration-Interaction (MR-CI) treatment that take advantage of the orbital locality. For large systems, the use of this formalism leads to a huge reduction of the computational costs, and permits MR-CI treatments that would have been impossibile by using the standard delocalized approaches. An applications of this technique to the study of mixed-valence organic systems will be presented. |
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