Ab Initio Studies of Small Molecules Using 1s Gaussian Basis Functions. III. LCGTO SCF MO Wavefunctions of the Three- and Four-Electron Systems He2+, He2, and Linear H3, H4+, H4

The ground states of the three‐ and four‐electron systems He₂⁺, He₂, and linear symmetric H₃, H₄⁺, and H₄ are investigated using the linear combinations of Gaussian‐type orbitals (LCGTO) self‐consistent‐field molecular‐orbital (SCF MO) approach. The basis functions are $\text{1s}$ GTO's, not necessarily located at nuclear positions. Optimized, near‐Hartree–Fock (HF) wavefunctions are presented for the isoelectronic H₃ and He₂⁺ systems, for which such results were not previously available. They are found to have similar correlation energies. With simpler wavefunctions, E‐vs‐$R$ plots for H₃ and He₂⁺ give good bond lengths and shapes of the energy curve. It is found that previous SCF MO studies have not closely approximated the true HF function for He₂. The H₄⁺ calculations are the first SCF MO results available for the species, and indicate its instability with respect to H₃⁺ + H, in agreement with recent experimental data. The GTO studies for H₄ are the only rigorously executed ab initio calculations yet done for the system. E‐vs‐$R$ data derived for H₄ refute the best previous studies, which used integral approximations. The interaction potential of two H₂ molecules (end to end) is derived from these H₄ energies.