MOLECULAR-DYNAMICS SIMULATIONS WITH FIRST-ORDER COUPLING TO A BATH OFCONSTANT CHEMICAL-POTENTIAL

In molecular dynamics simulations the temperature or pressure can be c ontrolled by applying a weak first-order coupling to a bath of constan t temperature or pressure. This weak coupling technique to control sys tem properties using a first-order relaxation equation is analyzed fro m a statistical mechanics point of view. It is shown, how the weak cou pling scheme can be generalized and applied to a bath of constant chem ical potential. The presented method, to which in the following will b e referred to as chemical potential weak coupling, is applied and test ed on a Lennard-Jones fluid. The thermodynamic quantities known from t he literature are accurately reproduced. The temperature and chemical potential weak coupling methods aim to sample the canonical and grand canonical ensembles respectively. By analyzing the fluctuations in ene rgy and number of particles, the tight relation between the ensembles and the distributions obtained from the weak coupling simulations is d emonstrated. The influence of the choice of the coupling parameters on the quality of the approximation of the ensemble distribution is disc ussed.