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.