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A Monte Carlo simulation shows that H2 molecules adsorbed on a NaCl(001) surface forms a commensurate c(2×2) structure where the hydrogen molecules sit flat on top of the cationic Na+ sites. The unit cell was found to have four molecules, where pairs of neighboring molecules are aligned perpendicular to each other in a "T" configuration. This structure is in agreement with the experimental results in terms of coverage and stability, but disagrees in terms of symmetry since the PIRS-ATR and HAS experimental results show a (1×1) structure. To solve this problem, the rotational motion of H2 molecules has been studied using perturbation theory and it is found that quantum effects will azimuthally delocalize the orientation of the molecular axis of H2. Thus, the c(2×2) structure becomes a (1×1) structure. Our simulations also show that a second layer is possible, where all hydrogen molecules adsorb over the anionic sites in a unit cell of p(2×1) symmetry. Perturbation Theory calculations show that p-H2 and o-H2 (J=1, m=±1) prefer to sit on the top of Na+ site, while o-H2 (J=1, m=0) prefers to locate over the site. |