First principles calculation of the potential energy surface for the lowest-quartet state of H-3 and modelling by the double many-body expansion method

We report a study of the potential energy surface for the lowest quartet st ate of H-3. At the ab initio level, restricted Hartree-Fock and full config uration interaction (FCI) calculations were performed with two extended Gau ssian basis sets providing a detailed coverage of the molecule configuratio n space. A total of 102 geometries, both linear and nonlinear, have been ex amined. These calculated energies have then been partitioned into two-body and three-body Hartree-Fock energy components, and combined with two-body a nd three-body semiempirical models of the dynamical correlation energy to o btain a realistic double many-body expansion (DMBE) representation of the t itle potential energy surface. In conjunction with a previously reported DM BE potential energy surface for the two lowest-doublet states of H-3, this completes the set of potentials on which accurate dynamics calculations may be carried out for any collision process involving three ground-state hydr ogen atoms. A number of FCI calculations have also been carried out to test the reliability of the modelled DMBE potential energy surface.