Ab initio rotation-vibration energies of H 3O +

Abstract

For the H 3O + molecular ion 47 points on the ground electronic state potential energy surface have been computed by configuration interaction calculations including all possible single and double excitations that can be generated from a canonical set of rather accurate Hartree-Fock SCF molecular orbitals. Combining these points with a set of 14 previously published points that were calculated in exactly the same way gives a good coverage of the surface. An analytic potential function has been fitted that involves all the vibrational coordinates to these points, and this involves the least squares optimization of the values of 13 parameters in the potential function. Using the nonrigid invertor Hamiltonian with this surface the low lying rotation-vibration energies have been calculated; it is hoped that these results will be of use in the search for the spectrum of H 3O +. The equilibrium bond length is found to be 0.978 Å, the equilibrium HOH bond angle is 111.6°, and the inversion barrier is 712 cm −1.