Ab initio calculation of the a 3Σ +u interaction potential and vibrational levels of 7Li2

The interaction potential of the lowest triplet excited state, a ³Σ ⁺_u, for Li ₂ has been calculated using the ab initio method, QCISD(T) and the correlation-consistent valence polarised quintuple zeta basis set, cc-pV5Z. Equilibrium constants and vibrational levels for ⁷Li₂ are in good agreement with experimental determinations. The a ³Σ ⁺_u energy curve for Li ₂ is characterised at the QCISD(T, full)/cc-pV5Z level of theory by the parameters D_e, D_o, R_e, ω_e and ω_ex_e which are found to be 334.145 cm ^-1, 301.989 cm ^-1, 4.1686 Å, 65.400 cm ^-1 and 3.208 cm ^-1, respectively.