Car-Parrinello simulation of NH3 adsorbed on the MgO(100) surface

The Car-Parrinello method has been applied the adsorption of ammonia on regular (100) surfaces of solid magnesium oxide. One NH3 molecule was positioned on a surface consisting of three layers with four Mg and four O atoms in each. The samples were first subjected to simulated annealing up to temperatures in the range of 50-300 K and then to a free molecular dynamics run covering time intervals of 2-3 ps. NH3 is physisorbed on the surface. The N atom is bound to the Mg but not centered above it. The dynamics of the NH3 molecule are discussed in terms of three modes, a stochastic reorientation of the molecule around its figure axis, a nutation around an axis vertical to the figure axis, and the N-H stretching modes. The calculation confirms recent inelastic neutron scattering results, which indicate rotation-translation coupling and a low barrier to the rotational motion.