MOLECULAR-DYNAMICS SIMULATION OF A POLYMER-CHAIN IN SOLUTION

Results of a molecular dynamics simulation of a single polymer chain i n a good solvent are presented. The latter is modeled explicitly as a bath of particles. This system provides a first-principles microscopic test of the hydrodynamic Kirkwood-Zimm theory of the chain's Brownian motion. A 30 monomer chain is studied in 4066 sol vent particles as w ell as 40/4056 and 60/7940 systems. The density was chosen rather high , in order to come close to the ideal situation of incompressible flow , and to ensure that diffusive momentum transport is much faster than particle motions. In order to cope with the numerical instability of m icrocanonical algorithms, we generate starting states by a Langevin si mulation that includes a coupling to a heat bath, which is switched of f for the analysis of the dynamics. The long range of the hydrodynamic interaction induces a large effect of finite box size on the diffusiv e properties, which is observable for the diffusion constants of both the chain and the solvent particles. The Kirkwood theory of the diffus ion constant, as well as the Akcasu et al. theory of the initial decay rate in dynamic light scattering are generalized for the finite box c ase, replacing the Oseen tensor by the corresponding Ewald sum. In lea ding order, the finite-size corrections are inversely proportional to the linear box dimensions. With this modification of the theory taken into account, the Kirkwood formula for the diffusion constant is verif ied. Moreover, the monomer motions exhibit a scaling that is much clos er to Zimm than to Rouse exponents (t2/3 law in the mean square displa cement; decay rate of the dynamic structure factor is-proportional-to k3). However, the prefactors are not consistent with the theory, indic ating that (on the involved short length scales) the dynamics is more complex than the simple hydrodynamic description suggests.