Molecular hydrogen in graphite: A path-integral simulation

Molecular hydrogen in the bulk of graphite has been studied by path-integral molecular-dynamics simulations. Finite-temperature properties of H-2 molecules adsorbed between graphite layers were analyzed in the temperature range from 300 to 900 K. The interatomic interactions were modeled by a tight-binding potential fitted to density-functional calculations. In the lowest-energy position, an H-2 molecule is found to be disposed parallel to the sheets plane. At finite temperatures, the molecule explores other orientations but its rotation is partially hindered by the adjacent graphite layers. Vibrational frequencies were obtained from a linear-response approach, based on correlations of atom displacements. For the stretching vibration of the molecule, we find at 300 K a frequency omega(s)=3916 cm(-1), more than 100 cm(-1) lower than the frequency corresponding to an isolated H-2 molecule. Isotope effects have been studied by considering also deuterium and tritium molecules. For D-2 in graphite we obtained omega(s)=2816 cm(-1), i.e., an isotopic ratio omega(s)(H)/omega(s)(D)=1.39.