COMPARISON OF MOLLER-PLESSET PERTURBATION-METHODS, COMPLETE ACTIVE SPACE SELF-CONSISTENT-FIELD THEORY, AND A NEW GENERALIZED MOLECULAR-ORBITAL METHOD FOR OXYGEN-ATOM TRANSFER FROM A MOLYBDENUM COMPLEX TO A PHOSPHINE

Complete active space self-consistent field (CASSCF), a new generalize d molecular orbital approach (GMO2), and Moller-Plesset (MP) perturbat ion calculations are compared to determine the reliable level of pertu rbation theory for the oxo-transfer reaction associated with oxomolybd enum enzymes. GMO2(FCI) reproduced the CASSCF results within 0.7 kcal/ mol for a small model system. Although GMO2(SDTQ) gave less reliable r esults (within 2 kcal/mol of the CASSCF results) than GMO2(FCI), GMO2( SDTQ) was used for larger systems because it is technically easier to perform. Comparing MP2, MP3, and MP4 to GMO2(SDTQ) revealed that MP2 a nd MP4 overestimated the stability of multiple M-O bonds. This overest imation is due to the low-lying orbitals in these multiply-bonded syst ems. Although these near-degenerate (nondynamical) correlation effects are handled better by variational methods, MP3 appears to be a reliab le level of perturbation theory for these ore-transfer reactions.