The FORTRAN 90 code and sample methane potential can be downloaded by clicking the link below. The primary citation for this code is the Wang et al. reference below.
Wang et al. J. Chem. Phys. 2008, 128, 224314. Our code
Anderson, J. Chem.Phys.1975,63, 1499. Theory and methodology
Anderson,.J. Chem. Phys.1976,65, 4121. Theory and methodology