Extending the functional form of the methane PES in redundant coordinates for highly excited vibrational energy levels calculation

Potential energy surfaces (PES) of methane which are constructed using ten symmetrised combinations of four bond length and six interbond angles are referred to as (10S) PESs. At high energy ranges, it was found that (10S) PESs permitted improving by factor of five the fit quality of ab initio electronic energies of methane versus standard nine symmetric coordinates (9S) PES representations both in internal, orthogonal or normal-mode coordinates. We extend predictions of vibrational band origins to spectral range above 10,000 cm⁻¹ to help future analyses of experimental spectra. The accuracy at high energies is increased without notable deterioration at low-E levels (rms deviation of 0.26 cm⁻¹). Based on a comparison between various variants of (10S) PESs we expect that new theoretical band origins should have the accuracy not worse than 1–3 cm⁻¹ up to 13,400 cm⁻¹.