Monte Carlo simulations show that, at one monolayer coverage, H-2 molecules adsorbed on a NaCl(0 0 1) surface occupy all Na+ sites and form a commensurate c(2 x 2) structure. If the Cl sites are occupied as well, a bi-layer p(2 x 1) structure forms. An examination of the H-2 molecules' rotational motion shows the molecular axes are azimuthally delocalized and so both of the structures acquire (1 x 1) symmetry in accord with experimental observations. These calculations also show that helicoptering o-H-2 (J = 1, m = +/- 1) prefer to sit on top of Na+ sites, while cartwheeling o-H-2 (J = 1, m = 0) prefers to locate over Cl sites, in agreement with other work. (C) 2008 Elsevier B. V. All rights reserved.