Theoretical Vibrational Energy Levels of H2 Associated with Various Combinations of Molecular-Orbital Configurations

A theoretical examination has been made of the influence of various MO configurations on the spacing of the vibrational levels of H₂. Eleven previously determined potential curves for the H₂ ground state, based upon LC STO MO wavefunctions with configuration interaction, have been used to calculate vibrational‐energy levels and their first differences ΔG. Comparison of the 11 curves of ΔG vs vibrational quantum number ν shows that only when the wavefunction contains the configuration (σ_u1sσ_u1s′) does the ΔG curve have a shape similar to that of the experimental ΔG curve. The combination of (σ_g1sσ_g1s′) and (σ_u1sσ_u1s′) provides left—right correlation or variable ionicity and this ionicity is found to vary in the same manner as the curvature of the ΔG curve. The addition of (σ_g2sσ_g2p), which gives in—out correlation, and (${\pi }_{\mathrm{u}}{\text{2pπ̄}}_{\mathrm{u}}\text{2p}$ ), which provides angular correlation, to the wavefunction raises the ΔG curve especially at large ν. The use of ΔG curves obtained from simple wavefunctions to estimate dissociation energies is discussed.