Accelerated Molecular Dynamics of Rare Events
S. Pal and K. A. Fichthorn, Depts. Of Chemical Engineering and Physics, Pennsylvania State University, University Park PA 16802
Diffusion of molecules in and on solid substrates often occurs as a series of hops between neighboring binding sites, or potential-energy minima. Simulation of this type of transport with molecular dynamics (MD) becomes challenging because the time between hops often exceeds times that can typically be probed with this computational technique. We discuss a method similar to that of Voter  to accelerate the dynamics where a much simpler form of the bias-potential is used. The emphasis of these methods is to accelerate the dynamics, while trying to retain kinetic detail. The computational overhead involved in our method is very small, and only a few lines of extra code needs to be incorporated in a standard MD code to implement this scheme.
To verify the algorithm, we use a model where an adatom hops on a two-dimensional potential-energy surface . The average potential energy of the adatom agrees with the value when no acceleration (boost) is applied. A more complicated model involving the diffusion of Ag clusters on the Ag(001) surface is studied. For an adatom, a study of the diffusion coefficient with temperature shows that boosts ranging from 200 to > 1010 can be achieved from room temperature to lower temperatures. We develop a model for cluster diffusion and a few small clusters are studied at room temperature. The diffusion of a larger cluster with 143 atoms is also studied, where the relative occurrence of different atomic moves are measured by spatial binning of the system. We find that among other types of motions, a large contribution to diffusion comes from the motion of periphery atoms. Mechanisms of periphery atom diffusion range from single-atom hopping to more complex motions where atoms at the periphery move in concert.
 A. F. Voter, J. Chem. Phys. 106, 4665 (1997).
 S. Pal, and K. A. Fichthorn (accepted for publication in the Chemical