Ab initio potential energy surface and rovibrational energies of H3O+ and its isotopomers

A new potential energy surface, based on high quality ab initio electronic structure calculations, is presented for the hydronium ion (H3O+). The new potential surface is used in rigorous calculations of vibrational energies of H3O+, D3O+, H2DO+, and HD2O+. Comparison with experiment shows significant improvement over our previous calculations using an earlier potential [X. Huang, S. C. Carter, and J. M. Bowman, J. Phys. Chem. B 106, 8182 (2002)]. Vibrational calculations are also presented with a new version of the code MULTIMODE. In this version the maximum number of coupled modes in the potential in any grouping of modes is increased from four (the previous maximum) to five. The importance of five-mode terms in the potential is demonstrated for several vibrational states in H3O+ and H2DO+. Also, in the new version of MULTIMODE the number of coupled modes in the Coriolis term can be varied independently from the number of coupled modes in the potential. Rovibrational calculations for J=1 are also presented for H3O+ and D3O+ and compared with experiment for H3O+.