Quantum Crystals Under Pressure as a Probe of Many-Body Forces

Though the concept of many-body forces has a clear-cut physical meaning and well-developed computational schemes exist to address this issue, there are few physical problems where the role of many-body interactions are unambiguous. The well-known problem of the preference of the heavier rare-gas solids to crystallize into a fcc lattice failed to give conclusive evidence that many-body forces have important implications beyond the renormalization of effective two-body potentials. The theory of lattice distortions in solid hydrogens and solid helium under pressure is developed, and it is shown that the equation of state of these systems may well be described with effective two-body potentials, whereas the lattice distortion parameter is very sensitive to the real nature of the intermolecular potential and thus can be used as a probe of the many-body forces.