High dimensional anharmonic potential energy surfaces: The case of methane

The overtone vibrational spectra of all T-d symmetry isotopomers of methane have been analyzed simultaneously. A Hamiltonian expressed in internal cur vilinear coordinates expanded to the fourth order has been employed, with a nine-dimensional basis of harmonic oscillator wave functions in symmetry c oordinates. Near-resonant anharmonic interactions are treated to first orde r, while weaker interactions are handled as second order perturbations. A s et of optimized Born-Oppenheimer force constants is obtained, which reprodu ces the observations up to 9500 cm(-1) and shows an excellent agreement wit h the results of ab initio calculations. (C) 1999 American Institute of Phy sics. [S0021-9606(99)01015-6].