GROUND-STATE AND LOWER EXCITED-STATE DISCRETE ABINITIO ELECTRONIC POTENTIAL-ENERGY SURFACES FOR DOUBLET HEH2+

Ab initio electronic energy calculations are reported for 596 nuclear configurations of HeH2+ (in Cs symmetry). The lowest four doublet spin-state eigenfunctions for HeH+2 were computed by diagonalizing a subset of the full-configuration interaction Hamiltonian matrix, selected by perturbation theory estimation, relative to a reference set of configurations including the Hartree-Fock configuration and all appropriate single excitation configurations. Trial wave functions corresponding to ground and excited states were constructed from 30 molecular orbitals expanded in a twice-double-zeta-plus-polarization contracted Gaussian basis. Two basis sets were employed: one constructed to produce greater accuracy for the ground-state (principal quantum number n equal to one) surface, and a second more contracted set in the n = 1 space and augmented with n = 2 basis functions to describe low-lying excited states. Absolute accuracy estimates of the ground- and excited-state surfaces are within 5 and 10 millihartree, respectively; relative errors are estimated to be less than 2-3 millihartree. © 1979 American Institute of Physics.