Ground- and lower excited-state discrete ab initio electronic potential-energy surfaces for doublet HeH2+

Ab initio electronic energy calculations are reported for 596 nuclear configurations of HeH2+ (in C/sub s/ symmetry). The lowest four doublet spin-state eigenfunctions for HeH2+ were computed by partially 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 occupations. 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