Ab initio calculation of the electronic structures of the 3Φ ground and 5Φ excited states of CoH

The electronic structures and the spectroscopic constants of the electronic ground (3)Phi and low-lying (5)Phi electronic excited states of the CoH molecule were studied by multireference single and double excitation configuration interaction (MR-SDCI)+Davidson's correction (Q) calculations and size-consistent multireference coupled pair approximation (MRCPA) calculations. Calculations were performed under C-infinity v symmetry using Slater-type basis functions. The electronic ground state was confirmed to be the (3)Phi state. It was found that at least four reference configurations were needed to describe the ground (3)Phi state correctly at the MR-SDCI+Q level, while the (5)Phi state can be described well by one reference configuration, namely, the Hartree-Fock configuration. Larger dynamical electron correlation for the low-spin (3)Phi state than that for the high-spin (5)Phi state is discussed. Spectroscopic constants, i.e., equilibrium bond lengths (r(e)), harmonic frequency (omega(e)), and excitation energy, obtained by the MR-SDCI+Q method showed good correspondence with experimental values. MRCPA calculations gave a slightly shorter value for r(e) than experimental values, but improved omega(e) and the excitation energy bringing them very close to experimental values. (c) 2007 American Institute of Physics.