			15min:

MR-SDCI + Q AB INITIO MOLECULAR ORBITAL CALCULATIONS OF FeCO: IMPORTANCE OF WELL-CONTRIVED SA-MCSCF WAVE FUNCTIONS AND 8-10 sigma

ELECTRON CORRELATIONS.

AMANO MICHIKO, SACHIKO S. ITONO AND TSUNEO HIRANO, Department of Chemistry, Faculty of Science, Ochanomizu University, Tokyo 112-8610, Japan; UMPEI NAGASHIMA, Tsukuba Advanced Computing Center, National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8562, Japan; MASAHIRO SEKIYA AND KIYOSHI TANAKA, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.

FeCO has been used as a benchmark molecule to evaluate the basis functions for Fe^, and the methods of calculation since it is known to be difficult to reproduce the energy difference between the a⁵ Sigma ^- and the X³ Sigma ^- states as well as the experimentally observed bond lengths of the X³ Sigma ^- state by ab initio molecular orbital calculations. We carried out the MR-SDCI + Q and MR-ACPF calculations, based on the state-averaged MCSCF orbitals, taking into account the electron-correlation of 8-10 sigma electrons with the active space consisting of Fe 3d, 4s orbitals and CO , ^* orbitals. Our predicted term value of the a⁵ Sigma ^- state, bond lengths r_e(Fe-C) and r_e(CO) of the X³ Sigma ^- state are 0.87 kcal~mol^-1, 1.720 Å, and 1.159 Å with relativistic energy corrections, which are to be compared with the corresponding experimental values of 3.24 kcal~mol^-1, 1.7270 Å~[r_s(Fe-C)],^d and 1.1586 Å~[r_s(CO)], respectively. Similar results have also been obtained by the MR-ACPF methods.