Basis set choice and basis set superposition error (BSSE) in periodic Hartree-Fock calculations on molecular crystals

Periodic ab initio Hartree-Fock calculations on molecular crystals using th e CRYSTAL programs are increasingly common, but to date basis set choice fo r these calculations seems to have been dictated more by convention rather than an informed choice from numerous alternatives. This work compares the use of basis sets built on 6-21G and 6-31G molecular basis sets with one do uble-zeta alternative. We show that the use of unmodified DZP basis sets is not only routinely possible, but results in substantially smaller basis se t superposition error in the estimation of lattice energies. Through atomic calculations we demonstrate that, in part, this can be attributed to inher ent deficiencies in the 6-21G and 6-31G basis sets, namely the constraining of valence s and p exponents to be equal. The analysis also sheds some lig ht on the frequently cited but often poorly understood numerical instabilit ies that occur in calculations on some molecular crystals with the 6-31G ba sis set.