Molecular dynamics simulations extended to various ensembles. I. Equilibrium properties in the isoenthalpic–isobaric ensemble

Equilibrium molecular dynamics simulations of the Lennard-Jones fluid have been performed to serve as tests of a method recently suggested by Andersen for obtaining results in the isoenthalpic–isobaric ensemble. The method involves the introduction of adiabatic volume fluctuations into the simulation by treating the volume as an additional dynamic variable having its own equation of motion. Our findings are that (a) it is a simple task to modify an existing molecular dynamics program to the new method and (b) results for the static properties of the fluid are the same as those obtained by other Monte Carlo and molecular dynamics techniques. At this stage the method still needs refinement in order to produce the correct dynamic properties of the fluid.