Three-body interactions in solid parahydrogen

We use coupled-cluster ab initio methods to evaluate the non-pairwise-additive interactions in clusters of three parahydrogen (pH2) molecules. For acute triangular (pH2)3 geometries that play a prominent role in solid pH2, these interactions lower substantially the trimer’s total interaction energy. Our findings suggest that a widely-used effective pair potential for solid pH2 derives its accuracy from a fortuitous cancellation of errors at small intermolecular distances.