The Stillinger-Weber model potential is applied to study the Si(001) 2 × 1 surface. Molecular dynamics computer simulation has used a cluster of 1280 atoms with periodic boundary conditions in two dimensions, the surface-vacuum interface being modelled by a slab configuration containing twenty atomic planes. It is shown that the model potential readily leads to a dimer model with substantial sub-surface relaxations. The model is free of any y-twist (direction normal to the dimerization direction) which was invoked by a LEED investigation. The electronic total energy calculations for the present model give significantly lower energy than a simple dimer model without sub-surface relaxations but not lower than the asymmetric buckled dimer model. The calculation also suggests that molecular dynamics with a model silicon potential may provide a useful starting point for a complex reconstruction which can be further imporved by other first principles methods. © 1985.