REI FUKUI, UMPEI NAGASHIMA AND TSUNEO HIRANO, Grid Technology Research Center, Institute of Advanced Industrial Science and Technology, 6-9-3 Ueno, Taito-ku, Tokyo 110-0015, Japan; HIROSHI TATEWAKI, Computation Center and Institute of Natural Sciences, Nagoya City University, Nagoya 467-8501, Japan; TAKESHI NORO AND KIYOSHI TANAKA, Division of Chemistry, Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, 060-0810, Japan.
The ground state of CoH has been determined experimentally as the 3 by the far-infrared magnetic resonance study, and theoretically by ab initio methods.\footnoteD.P. Chong, S.R. Langhoff, C.W. Bauschlicher, Jr., S.P. Walch, and H. Partridge, J. Chem. Phys ., 85, 2850-2860 (1986);
M. Freindorf, C. M. Marian, and B. A. Hess, J. Chem. Phys ., \textbf99, 1215-1223 (1993). However, MR-SDCI and MR-SDCI + Q ab initio molecular orbital calculations predicted that the lowest electronic state is 5 .~ The size-consistent MR-ACPF method correctly predicted that 3 should be the ground state.~ As is previously reported by Tanaka et al. on FeH, the controversial results in MR-SDCI may arise through truncation error inherent in the size-inconsistent MR-SDCI method.
Another difficulty is that we cannot clearly separate the and states when CoH is treated under C2v symmetry, not under its real symmetry of C v. The best results we obtained at the moment are: re = 1.528 Å, Be = 7.291 cm- 1, and e = 2005 cm- 1 at the level of the MR-ACPF + relativistic correction using the Roos ANO (Co) and Dunning aug-cc-pVQZ (H) basis sets.