Importance of anisotropic three-body forces in solid hydrogen

We take into account three-body anisotropic forces between molecules to calculate the energy of the S00 triplet in solid hexagonal close packed hydrogen under pressure. Three-body contributions result in one term depending on the orientation of only one molecule crystal field term and two others that couple rotations of different molecules roton terms. Three-body interactions contribute, to a large extent, to the roton frequencies. Their inclusion in the calculation increases the calculated average frequency of the triplet, even at relatively low density, changing substantially the estimate of the internuclear distance. By contrast, the triplet splitting is substantially unaffected by three-body terms, resulting therefore a good candidate to test anisotropic two-body potential models against experiment.