Variational Calculations of Rotational−Vibrational Energies of CH4 and Isotopomers Using an Adjusted ab Initio Potential

We report variational calculations of vibrational energies of CH 4, CH 3 D, CH 2 D 2, CHD 3, and CD 4 using the code “Multimode” and the ab initio force field of Lee and co-workers [Lee, T. J.; Martin, J. M. L.; Taylor, P. R. J. Chem. Phys . 1995, 102, 254], re-expressed using Morse variables in the stretch displacements. Comparisons are made with experimental energies for CH 4 with this potential, and then small adjustments are made to the potential to improve agreement with experiment for CH 4 . Calculations for the isotopomers are done using the adjusted potential and compared with experiment. Additional vibrational energies and assignments not reported experimentally are also given for CH 4 and the isotopomers. Exact rotational−vibrational energies of CH 4 are also reported for J = 1.