Configuration-Interaction Study of the H3− System. II. Expanded Basis

A configuration‐interaction calculation on the ground state of H₃⁻ has been carried out using an orbital basis set made out of $\text{1s, 1s′, 2s}$ , and $\text{2p}$ STO's. The new orbitals were included in the set one type at a time and a large number of calculations was performed to optimize the nonlinear parameters and to keep the wavefunction from becoming too complicated while including the most important configurations that can be formed from each basis set. The best wavefunction obtained from each set was analyzed in terms of natural orbitals (NO). The first two NO's were used to construct simple geminals and study H₃⁻ in terms of separated pairs. The Rank 1 functions were used to estimate the Hartree–Fock energy for the system. Electron population analysis was done to gain information about the distribution of electrons in H₃⁻. Information on other four‐electron systems (He₂ and LiH) was used in the discussion of the results obtained for H₃⁻.