The molecular potential energy surface and vibrational energy levels of methyl fluoride. Part II

Abstract

New analytical bending and stretching, ground electronic state, potential energy surfaces for CH₃F are reported. The surfaces are expressed in bond-length, bond-angle internal coordinates. The four-dimensional stretching surface is an accurate, least squares fit to over 2000 symmetrically unique ab initio points calculated at the CCSD(T) level. Similarly, the five-dimensional bending surface is a fit to over 1200 symmetrically unique ab initio points. This is an important first stage towards a full nine-dimensional potential energy surface for the prototype CH₃F molecule. Using these surfaces, highly excited stretching and (separately) bending vibrational energy levels of CH₃F are calculated variationally using a finite basis representation method. The method uses the exact vibrational kinetic energy operator derived for XY₃Z systems by Manson and Law (preceding paper, Part I, Phys. Chem. Chem. Phys., 2006, 8, DOI: 10.1039/b603106d). We use the full C_3v symmetry and the computer codes are designed to use an arbitrary potential energy function. Ultimately, these results will be used to design a compact basis for fully coupled stretch–bend calculations of the vibrational energy levels of the CH₃F system.