Nonequilibrium molecular dynamics simulations of heat flow in one-dimensional lattices

We study the use of the Evans nonequilibrium molecular dynamics (NEMD) heat flow algorithm for the computation of the heat conductivity in one-dimensional lattices. For the well-known Fermi-Pasta-Ulam model, it is shown that when the heat field strength is greater than a certain critical value (which depends on the system size) solitons can be generated in molecular dynamics simulations starting from random initial conditions. Such solitons are stable and travel with supersonic speeds. For smaller heat fields, no solitons are generated in the molecular dynamics simulations; the heat conductivity obtained via the NEMD algorithm increases monotonically with the size of the system.

Microscopic expressions for the thermodynamic temperature

We show that arbitrary phase space vector fields can be used to generate phase functions whose ensemble averages give the thermodynamic temperature. We describe conditions for the validity of these functions in periodic boundary systems and the molecular dynamics (MD) ensemble, and test them with a short-ranged potential MD simulation.

Configurational temperature for systems with constraints

A general method is developed for constructing configurational expressions for the temperature of systems with constraints. As an example, this method is applied to molecular systems with bonding constraints, and an explicit formula for the temperature, in terms of only the configurational variables of the system, is derived. This formula is tested against molecular-dynamics simulations for freely jointed Lennard-Jones 8-mer chains and Monte Carlo simulations for a system of diatomic Lennard-Jones molecules.