Molecular dynamics simulation and energy minimization of O2 adsorbed on a graphite surface

Simulation studies of O₂ on graphite show that the quadrupole interaction strongly affects the orientational correlation between neighbouring molecules in the adsorbed solid. The energy minimization study shows two non-equivalent competing structures with nearest neighbours arranged parallel and at an angle of 60°. The precise value of the quadrupole moment stabilises one structure with respect to the other. For Θ = 0 the simulated structure agrees with experiment. For values of the quadrupole close to the experimental value there is a significant discrepancy between experiment and theory. This discrepancy can be understood in terms of the neglect of the magnetic forces arising from the exchange interaction. The best effective σ₀₀ is 5% higher than the value recommended by bulk solid studies. The minimum energy structure is incommensurate and molecules are aligned preferentially parallel to the surface at the coverage in this study.