Inherent structures in the potential energy landscape of solid 4He

Abstract

We study the potential energy landscape (many-atom potential energy as a function of atomic positions) of solid hcp 4He in the vicinity of the 0 K crystal structure using an accurate pair potential. At the melting point, the potential energy of the helium lattice is far above the minimum hcp interatomic potential energy. We confirm previous conclusions (based on less accurate potentials) that all of the classical phonon frequencies at the 0 K melting pressure are imaginary, indicating that the melting-point crystal corresponds to a local maximum in the potential landscape; a pressure of about 1300 bar, however, makes it a local minimum. We find that the atomic arrangements that lie at local minima in the potential landscape (‘‘inherent structures’’) are glassy and porous, and have much lower potential energy than the crystalline form at the same density. We have quantitatively characterized the glassy structures by their radial distribution functions and coordination number distributions; they qualitatively resemble inherent structures for classical monatomic liquids, but exhibit differences of detail. A model variational calculation has been carried out for the melting-density ground state. It utilizes separate basis functions for each of the inherent structures, predicts a large Lindemann ratio for the crystal, and indicates that the probability distribution is a maximum at the perfect lattice configuration.