Ab initio calculations on the spectroscopic constants, vibrational levels and classical turning points for the 21Πu state of dimer 7Li2

The accurate dissociation energy and harmonic frequency for the highly excited 2(1)Pi(u) state of dimer Li-7(2) have been calculated using a symmetry-adapted-cluster configuration-interaction method in complete active space. The calculated results are in excellent agreement with experimental measurements. The potential energy curves at numerous basis sets for this state are obtained over a wide internuclear separation range from about 2.4a(0) to 37.0a(0). And the conclusion is gained that the basis set 6-311++G(d,p) is a most suitable one. The calculated spectroscopic constants D-e, R-e, omega(e), omega(e)chi(e), alpha(e) and B-e at 6-311++ G(d,p) are 0.9670 eV, 0.3125 nm, 238.6 cm(-1), 1.3705 cm(-1), 0.0039 cm(-1) and 0.4921 cm(-1), respectively. The vibrational levels are calculated by solving the radial Schrodinger equation of nuclear motion. A total of 53 vibrational levels are found and reported for the first time. The classical turning points have been computed. Comparing with the measurements, in which only the first nine vibrational levels have been obtained so far, the present calculations are very encouraging. A careful comparison of the present results of the parameters D-e and omega(e) with those obtained from previous theories clearly shows that the present calculations are much closer to the measurements than previous theoretical results, thus representing an improvement on the accuracy of the ab initio calculations of the potentials for this state.