Recently, the synthesis of ammonia over ruthenium-based catalysts has become an industrially viable process. Unfortunately, investigations of ammonia synthesis over ruthenium are scarce, particularly in comparison to the number of studies carried out over iron. To begin to fill this void, we have performed a series of electronic density-functional theory (DFT) calculations to investigate the effect of particle size and surface structure on ammonia synthesis over ruthenium. Our study has focused on the dissociative adsorption of dinitrogen, which is thought to be the rate-determining step in the synthesis, on both single-crystal surfaces and spherical clusters of ruthenium. The equilibrium adsorbate geometries were remarkably similar on both the single-crystal surfaces and the spherical clusters studied. The binding energy of dinitrogen in the end-on state exhibited a strong dependence on ruthenium surface atom coordination, being much stronger on atoms with low coordination. The main difference between the two single-crystal surfaces studied was the ability of the open Ru(11 (2) over bar 0) face to stabilize a low-energy side-on dinitrogen state, while the close-packed Ru(0001) face could not. It is likely that this stable Side-on state provides a low-energy dissociation pathway.