MOLECULAR-DYNAMICS ALGORITHMS AND HYDRODYNAMIC SCREENING

In this paper molecular dynamics simulations of a system of Brownian p articles in an explicit bath of solvent particles are considered. Gene ralized algorithms (Langevin simulations), in which both the Brownian particles and the solvent particles are artificially coupled to a heat bath, are analyzed for their dynamical properties on long length scal es. Although such a dynamic is clearly unphysical, its analysis is use ful for two reasons: The Langevin algorithm is frequently applied in a n ad hoc fashion, and the deviation of its dynamical properties from t he physical Hamiltonian case can be made arbitrarily small by choosing a sufficiently weak coupling to the heat bath. By a direct applicatio n of the Mori-Zwanzig projection operator formalism it is shown that t he violation of global momentum conservation results in an artificial screening of the hydrodynamic interactions, with a screening length pr oportional to the inverse square root of the friction constant of the algorithm. The result is formally similar to expressions given in phen omenological theories of hydrodynamic screening in semidilute polymer solutions.