title: Calculation of converged rovibrational energies and partition function for methane using vibrational-rotational configuration interaction.
abstract: The rovibration partition function of CH4 was calculated in the temperature range of 100-1000 K using well-converged energy levels that were calculated by vibrational-rotational configuration interaction using the Watson Hamiltonian for total angular momenta J = 0-50 and the MULTIMODE computer program. The configuration state functions are products of ground-state occupied and virtual modals obtained using the vibrational self-consistent field method. The Gilbert and Jordan potential energy surface was used for the calculations. The resulting partition function was used to test the harmonic oscillator approximation and the separable-rotation approximation. The harmonic oscillator, rigid-rotator approximation is in error by a factor of 2.3 at 300 K, but we also propose a separable-rotation approximation that is accurate within 2% from 100 to 1000 K.
authors.affiliation: Department of Chemistry, University of Minnesota, Minneapolis 55455-0431, USA. chakra@t1.chem.umn.edu
authors.forename: Arindam
authors.initials: A
authors.lastname: Chakraborty
authors.forename: Donald G
authors.initials: DG
authors.lastname: Truhlar
authors.forename: Joel M
authors.initials: JM
authors.lastname: Bowman
authors.forename: Stuart
authors.initials: S
authors.lastname: Carter