The vibrational structure of H+4 and D+4

Abstract

A potential energy surface (PES) for the H+4 system which is a fit to local, high quality ab initio multiple reference single and double excitations configuration interaction (CI) calculations (142 data points) is reported. The potential energy surface obtained here has been calculated by maintaining H+3 as a core in its equilibrium geometry and moving the remaining hydrogen atom around it (three-dimensional potential energy surface). The new surface supposes an improvement on the preceding potential energy surfaces considering both the basis set size and the ab initio method used here. The energy minimum of the potential presented here has been found to be about 2.5 kcal/mol lower than those obtained in previous studies, indicating that H+4 and D+4 are more stable ions than previously believed. A three-dimensional vibrational kinetic energy operator in internal coordinates without singularities has been derived. Energy and wave functions of the vibrational levels of the dissociating hydrogen in H+4 and D+4 systems have been calculated by using the derived potential and kinetic energy operators and integrating the vibrational Hamiltonian with the normal coordinates finite elements method. The vibrational states with energies below the new dissociation limit are reported and characterized, giving a more complete description of the vibrational structure. The number of bound vibrational levels obtained here is 7 for H+4 and 24 for D+4.