Estimation of the density of states by multicanonical molecular dynamics simulation

We formulate a procedure for calculating the density of states (DOS) from a multicanonical molecular dynamics (MMD) simulation. DOS cannot be obtained directly from the result of MMD simulation, because the Gaussian thermostat that is used in MMD simulation restricts the system to a spherical surface in momentum space. We perform MMD simulation for liquid Ar with Lennard-Jones potentials and evaluate DOS. Some physical quantities are estimated as a function of temperature from that DOS. The internal energy, entropy, and Helmholtz free energy are in good agreement with experiment. The quantity related to the fluctuation-the specific heat at constant volume-does not agree with experiment, which is ascribed to insufficient accuracy of DOS.