The widely used Stillinger-Weber potential for silicon interactions has been modified to provide an accurate description of the Si(111)-(7 x 7) surface including the highly-reactive adatom and rest-atom sites. This modified potential also provides a good representation of bulk silicon, and the Si(001)-(1 x 1), Si(001)-(2 x 1), Si(111)-(1 x 1) and Si(111)-(2 x 1) surfaces. Above the melting temperature of 1683 K, however, the original Stillinger-Weber potential is probably superior.