The adsorption and dissociation of molecular water on the defective (001) surface of galena (PbS) have been investigated using both periodic and embedded cluster electronic structure methods. The results show that in the periodic calculations, the energies are particularly sensitive to the thickness of the simulated slab. The predicted activation and reaction energies from the different approaches are in good agreement given the different levels of theory used, and show that water will easily dissociate at a surface step site, with the product structure being the more stable in such an environment.