Vibrational energy levels for CH4 from an ab initio potential

Many areas of astronomy and astrophysics require an accurate high temperatu re spectrum of methane (CH,). The goal of the present research is to determ ine an accurate ab initio potential energy surface (PES) for CH,. As a firs t step towards this goal, we have determined a PES including up to octic te rms. We compare our results with experiment and to a PES based on a quartic expansion. Our octic PES gives good agreement with experiment for all leve ls, while the quartic PES only for the lower levels. (C) 2001 Published by Elsevier Science B.V.