We performed CAS-CI calculations on Li2 using a set of molecular orbitals (MO) optimized with a procedure that, in the case of highly symmetric molecules, permits extraction of a small set of MO out of a large set of atomic orbitals (AO). The dimension of the CAS-CI space was of about 12 million symmetry-adapted determinants. We determined some spectroscopic constants of Li2 with three different atomic basis sets of increasing quality. The values obtained with the largest atomic basis set are very close to the experimental results.