Nature of the many-particle potential in the monatomic liquid state: Energetics, kinetics, and stability

Abstract

Molecular-dynamics calculations have been used to explore and characterize the many-particle potential underlying the motion of particles in the monatomic liquid state. The potential used accurately represents metallic sodium at the density of the liquid at melt. It is found that the potential surface is composed of a large number of stable nearly harmonic valleys, and that these can be classified as random, symmetric, or crystalline. The random valleys cover by far the major portion of configuration space; they are macroscopically uniform, i.e., they all have the same structural potential and vibrational spectrum; and they all have microscopically irregular anharmonicity. The symmetric valleys lie at potential energies below the random valleys, but above the bcc crystalline valley. The symmetric valleys are not macroscopically uniform, but show scatter in their structural potentials and their eigenvalue spectra, and the symmetric valleys also have microscopically irregular anharmonicity. The equilibrium states of our system, from zero temperature up to and including the liquid states, fall into three groups, random, symmetric, and crystalline, according to which class of potential valley is mainly visited in the system motion. The random states are well separated from the symmetric and crystalline states, on the graph of mean potential energy versus temperature. The random states lie on a single line over the entire temperature range, and they include the liquid states, demonstrating that the random valleys dominate the statistical mechanics of the liquid. The present results provide detailed confirmation of the liquid-dynamics Hamiltonian previously used in equilibrium and nonequilibrium calculations. Further, the liquid-dynamics prediction of near equality of the log moment of the vibrational spectra, for the liquid and crystal at the same density, is verified here for the example of sodium.