Calculation of converged rovibrational energies and partition function for methane using vibrational-rotational configuration interaction

Chakraborty, A., Truhlar, D.G. , Bowman, J.M. and Carter, S. (2004) Calculation of converged rovibrational energies and partition function for methane using vibrational-rotational configuration interaction. Journal of Chemical Physics, 121 (5). pp. 2071-2084. ISSN 0021-9606 doi: 10.1063/1.1759627

Abstract/Summary

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. (C) 2004 American Institute of Physics.