ROVIBRATIONAL ENERGY-LEVELS AND EQUILIBRIUM GEOMETRY OF HCP

The ground state potential energy surface for HCP has been investigate d theoretically, A large fraction of electron correlation is included by multireference internally contracted configuration interaction from CASSCF reference wave functions using large orbital expansions. The o rigin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for the four isotopicall y substituted species. Variational calculations of vibrational and rot ational frequencies for transitions up to J = 7 <-- 6 have finally bee n performed, with accuracy which is typically +/- 5 cm(-1) for vibrati ons and +/- 10 MHz for most rotations. By comparison with the results of the perturbation treatment the importance of the nu(1):2 nu(3) Ferm i interaction for vibrational frequencies and effective rotational con stants has been determined. From computed and experimental ground stat e rotational constants, the molecular equilibrium geometry has also be en estimated. (C) 1996 American Institute of Physics.