Rotational and vibrational energy levels of methane calculated from a new potential energy surface

A new potential energy surface of methane is constructed using extended ab initio CCSD(T) calculations at 19 882 points. Its analytical representation is determined through an expansion in symmetry adapted products of orthogonal coordinates involving 276 parameters up to 8th order with the equilibrium bond re = 1.08601 ± 0.00004 A and four quadratic parameters scaled to experimental fundamental vibration frequencies. Variational calculations give RMS (obs.-calc.) deviations of 0.085 and 0.25 cm−1 for vibration levels of the pentad and octad. Rotational energies up to J = 10 are calculated using potential expansion in normal coordinate tensors with maximum errors of 0.0007 and 0.0003 cm−1 for 12CH4 and 13CH4.