An accurate ab initio HOCl potential energy surface, vibrational and rotational calculations, and comparison with experiment

Accurate ab initio multireference configuration interaction (CI) calculations with large correlation-consistent basis sets are performed for HOCl. After extrapolation to the complete basis set limit, the ab initio data are precisely fit to give a semiglobal three-dimensional potential energy surface to describe HOCl-->Cl+OH from high overtone excitation of the OH-stretch. The average absolute deviation between the ab initio and fitted energies is 4.2 cm(-1) for energies up to 60 kcal/mol relative to the HOCl minimum. Vibrational energies of HOCl including the six overtones of the OH-stretch are computed using a vibrational-Cl method on the fitted potential and also on a slightly adjusted potential. Near-spectroscopic accuracy is obtained using the adjusted potential; the average absolute deviation between theory and experiment for 19 experimentally reported states is 4.8 cm(-1). Very good agreement with experiment is also obtained for numerous rotational energies for the ground vibrational state, the ClO-stretch fundamental, and the fifth overtone of the OH-stretch. (C) 1998 American Institute of Physics.