First principles calculation of the potential energy surface for the lowest-quartet state of H3 and modelling by the double many-body expansion method

Abstract

We report a study of the potential energy surface for the lowest quartet state of H3. At the ab initio level, restricted Hartree–Fock and full configuration interaction (FCI) calculations were performed with two extended Gaussian basis sets providing a detailed coverage of the molecule configuration space. A total of 102 geometries, both linear and nonlinear, have been examined. These calculated energies have then been partitioned into two-body and three-body Hartree–Fock energy components, and combined with two-body and three-body semiempirical models of the dynamical correlation energy to obtain a realistic double many-body expansion (DMBE) representation of the title potential energy surface. In conjunction with a previously reported DMBE potential energy surface for the two lowest-doublet states of H3, this completes the set of potentials on which accurate dynamics calculations may be carried out for any collision process involving three ground-state hydrogen atoms. A number of FCI calculations have also been carried out to test the reliability of the modelled DMBE potential energy surface.