Ab initiosimulation of structure in amorphous hydrogenated carbon

First-principles quantum molecular-dynamics simulations of the structure of hydrogenated amorphous carbon, a-C:H, at two densities (2.0 and 2.9 g/cm³) have been carried out using the Car-Parrinello method. The results for the low-density structure show good agreement with experiment in the manner in which the hydrogen is incorporated, as judged by agreement with published vibrational density of states and neutron-diffraction data at various levels of deuteration. The simulation reproduces the position and magnitude of the hydrogen features in the pair-correlation function $G(r)$ obtained from neutron diffraction. The nonhydrogenated carbon atoms are predominantly sp² hybridized and the hydrogen atoms are largely ``sandwiched'' between layers of sp² atoms. The simulated high-density structure has a majority of nonhydrogenated carbon atoms with sp³ hybridization. The results of this study show that a useful test for confirming the high-density a-C:H structure is the presence of a small C-C-C bond angle and the occurrence of C-H bond stretching frequencies above 3100 cm^-1.