Regarding the 6-31G basis in G86: The 2S and 3S are the inner and outer functions used to give more flexibility to the 2S character of the first row atoms. In the same way if you use 6-311G, you will get 2S, 3S, and 4S for the triple split 2S character of the same atoms. The numbers are used to facilitate bookeeping. Hope this help Jorge M. Seminario