EFFICIENT TRUNCATION STRATEGIES FOR MULTIREFERENCE CONFIGURATION-INTERACTION MOLECULAR-ENERGIES AND PROPERTIES

Modified virtual orbitals are proposed for multi-reference configurati on interaction (MRCI) treatments and a modified Fock operator is defin ed for the orbital transformation. The main property of the modified o rbitals is to improve the convergence properties of the configuration interaction (CI) expansion, which can be exploited to truncate? partia lly, the expansion in the external space. Simple tests are presented t o show that the orbital transformation may be useful to perform FullCI type of treatments for subsets of orbitals and electrons, and to impr ove the MRCI second-order corrections and energies. Compared to other well-established techniques for accurate MRCI treatments, it is believ ed that this method offers advantages for electronic structures with m any active orbitals and electrons using large orbital basis sets. (C) 1998 American Institute of Physics. [S021-9606(98)01341-5].