(184f) A Superbasin Kinetic Monte Carlo Method

A ubiquitous problem in atomic scale simulation of materials is the small barrier problem, in which the energy landscape presents “superbasins” with low intra-basin energy barriers relative to the inter-basin barriers.  Rare-event simulation methods, such as kinetic Monte Carlo (KMC) and accelerated molecular dynamics, are inefficient for such systems because considerable effort is spent in simulating short-time, intra-basin motion without evolving the system significantly.  We have developed a method for treating fast, intra-basin motion using a Master-Equation approach to obtain net, inter-basin escape rates.  These escape rates are then incorporated into KMC simulations to reach significantly longer times than could be probed otherwise.

 We present two different criteria for defining superbasins on the fly within a KMC simulation, then we discuss and apply our Master Equation solution strategy to model systems with superbasins that have various attributes.  As the final application of our method, we use it to model Ga adatom diffusion on the beta 2(2x4) reconstruction of GaAs(001).  The diffusion of Ga is a complex phenomenon involving many local minima.  According to an analytic bond-order potential, up to 23 local minima can be involved in motion between two nearest-neighbor binding sites – although this number is less when first-principles density-functional theory is used to quantify the minima.  We find that the diffusion of Ga involves climbing many small energy barriers to escape a superbasin and it is a classic example of the small-barrier problem.  We simulate Ga adatom diffusion using our approach and achieve accelerations of up to ten orders of magnitude compared to a conventional KMC simulation.