Using experimental data and a contracted basis Lanczos method to determine an accurate methane potential energy surface from a least squares optimization

We obtain an accurate methane potential energy surface (PES) by starting with the ab initio PES of Schwenke and Partridge [Spectrochim. Acta A 57, 887 (2001)] and adjusting 5 of their parameters to reproduce 40 reliable experimentally determined vibrational levels of CH4. The 40 levels include all 35 levels in polyads up to and including the Octad and 5 levels in the Tetradecad. The Tetradecad levels are obtained from direct experimental transitions. The fit reduces the root mean square deviation of these 40 levels from 4.80 cm(-1) to 0.28 cm(-1). The new PES ought to aid in the analysis of the Tetradecad. To further test the accuracy of the new PES, vibrational levels are computed for CH4, CH3D, CHD3, and (CH4)-C-13 and are compared with the extensive experimental data. The errors are all within about 1 cm(-1) except for a few cases. (C) 2014 AIP Publishing LLC.