Ab initio calculation of the ground and first excited states of the lithium dimer

Based on a high level ab initio calculation which is carried out with the multireference configuration interaction method under the aug-cc-pVXZ (AVXZ) basis sets, X = T, Q, 5, the accurate potential energy curves (PECs) of the ground state (XEg+)-E-1 and the first excited state A(1)E(u)(+) of Li-2 are constructed. By fitting the ab initio potential energy points with the Murrell- Sorbie potential function, the analytic potential energy functions (APEFs) are obtained. The molecular bond length at the equilibrium (R-e), the potential well depth (D-e), and the spectroscopic constants (B-e, omega(e), alpha(e), and omega(e)chi(e)) for the (XEg+)-E-1 state and the A(1)E(u)(+) state are deduced from the APEFs. The vibrational energy levels of the two electronic states are obtained by solving the time-independent Schrodinger equation with the Fourier grid Hamiltonian method. All the spectroscopic constants and the vibrational levels agree well with the experimental results. The Franck-Condon factors (FCFs) corresponding to the transitions from the vibrational level (v' = 0) of the ground state to the vibrational levels (v '' = 0-74) of the first excited state have been calculated. The FCF for the vibronic transition of A(1)E(u)(+) (v '' = 0) <-(XEg+)-E-1 (v' = 0) is the strongest. These PECs and corresponding spectroscopic constants provide reliable theoretical references to both the spectroscopic and the molecular dynamic studies of the Li-2 dimer.