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

The converged rovibrational energies and partition function for methane were discussed using vibration-rotational configuration interaction. It was found that the configuration state functions were product of ground-state occupied and virtual modals obtained using the vibrational self-consistent field method. It was observed that the partition function was used to test the harmonic oscillator approximation and the separable-rotation approximation. A separable-rotation approximation that was accurate within 2% from 100 to 1000 K was also presented.