Excitation energy dedicated molecular orbitals. Method and applications tomagnetic systems

This paper proposes a general strategy to define molecular orbitals which a re especially adapted to the calculation of the energy difference between t wo states. These orbitals an shown to be eigenvectors of blocks of the diff erence between the density matrices relative of the two states. They may be used for rational enlargement of the active space in CASSCF calculations o r for truncations of the configuration interaction space. Several examples show the relevance of the method to identify the few molecular orbitals of a bridge between magnetic centers which play a role in the spin coupling me chanism.