2ND-ORDER MOLLER-PLESSET PERTURBATION-THEORY FOR SYSTEMS INVOLVING FIRST TRANSITION ROW METALS

Results obtained using second-order Moller-Plesset (MP2) perturbation theory are compared with those obtained using coupled cluster singles and doubles approach including a perturbational estimate of the connec ted triple excitations, CCSD(T), or the modified coupled-pair function al (MCPF) approach. For FeCO, FeCO+, FeH2O+, and the states of FeCH4derived from the Fe+ 3d(6)4s(1) occupation, the MP2 geometries are fou nd to be in good agreement with those obtained at the CCSD(T) or MCPF level. At the MP2 level the Fe-C bond lengths for the states of FeCH4 derived from the 3d(7) occupation are too long relative to the MCPF o r CCSD(T) results. The geometries computed at the self-consistent-fiel d (SCF) level are quite poor for FeCO+ and all states of FeCH4+. The e nergetics at the MP2 level, while superior to the SCF, are still signi ficantly inferior to those obtained at the CCSD(T) or MCPF level. We f ind that FeCH4+ has a quartet ground state with an eta(3) coordination .