Ground- and lower excited-state discrete a b i n i t i o electronic potential-energy surfaces for doublet HeH2+a)

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Abstract
Ab initio electronic energy calculations are reported for 596 nuclear configurations of HeH2+ (in Cs 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.