Methane high-temperature partition function from contact transformations and variational calculations

The temperature dependence of the methane ¹²CH₄ partition function was investigated up to T=3000K. The ro-vibrational energy levels were calculated using high-order contact transformation (CT) method that permitted building accurate effective Hamiltonian models for highly excited vibration-rotation states from the molecular potential energy surface. Comparisons with variational calculations using large basis sets as well as convergence and extrapolation issues are discussed. The accuracy of the present calculations for the methane partition function was estimated as 1-2% at 2000K. This estimated error is considerably smaller than the difference with other calculations available in the literature.