Abstract Small MoS 2 slabs, which are the active component of hydrotreating catalysts, have been modelled by molecular graphics and molecular mechanics. Active sites at molybdenum are created by removing edge sulphur atoms. The structures and energies of slabs having molybdenum atoms with different degrees of coordinative unsaturation have been investigated. Structures where two molybdenum atoms are not linked by at least one sulphur atom are unstable owing to repulsion between the molybdenum atoms. They distort to increase the distance between the molybdenum atoms. Fully saturated slabs (those which terminate with SH groups) are also strained because of repulsions between the SH groups.