CONFIGURATION-INTERACTION CALCULATIONS WITH TERMS LINEAR IN THE INTERELECTRONIC COORDINATE FOR THE GROUND-STATE OF H3- A BENCHMARK STUDY( )

The CISD-R12 method is applied to the ground states of H-2 and H-3+ at their equilibrium configurations, with both bare nuclear Hamiltonian (BNH) and self-consistent field (SCF) references with eight basis sets that range from 10 s (i.e., ten contracted s-type Gaussians) to 30s20 p12d9f. With the largest basis set, the energy of H-2 is obtained with an error of 2muE(h), while for H-3+, the error is probably smaller an d the best calculated energy of -1.343 835E(h) is supposed to be accur ate to all indicated figures. The BNH reference is always superior to the SCF reference. The relativistic corrections are evaluated at the S CF level by means of ''direct perturbation theory'' both for H-2 and H -3+. The Hylleraas-SCF methods and related concepts for the constructi on of an optimum reference function are discussed in the Appendix.