Off-Lattice Self-Learning Kinetic Monte Carlo: Application to 2D Cluster Diffusion on the fcc(111) Surface

TL;DR

This paper introduces an off-lattice self-learning kinetic Monte Carlo method that enhances the modeling of atomic diffusion on metal surfaces by allowing atoms to occupy off-lattice positions, revealing new atomic mechanisms.

Contribution

The paper develops an off-lattice extension of the self-learning KMC method, enabling more accurate simulation of atomic diffusion processes on metal surfaces.

Findings

Revealed new atomic mechanisms for cluster migration.

Successfully applied to 2D Cu island diffusion on Cu and Ag(111).

Uncovered mechanisms like shear, breathing, and occupancy changes.

Abstract

We report developments of the kinetic Monte Carlo (KMC) method with improved accuracy and increased versatility for the description of atomic diffusivity on metal surfaces. The on-lattice constraint built into our recently proposed Self-Learning KMC (SLKMC) [1] is released, leaving atoms free to occupy Off-Lattice positions to accommodate several processes responsible for small-cluster diffusion, periphery atom motion and hetero-epitaxial growth. The technique combines the ideas embedded in the SLKMC method with a new pattern recognition scheme fitted to an Off-Lattice model in which relative atomic positions is used to characterize and store configurations. Application of a combination of the drag and the Repulsive Bias Potential (RBP) methods for saddle points searches, allows the treatment of concerted cluster, and multiple and single atom motions on equal footing. This tandem approach has helped reveal several new atomic mechanisms which contribute to cluster migration. We present applications of this Off-Lattice SLKMC to the diffusion of 2D islands of Cu (containing 2 to 30 atoms) on Cu and Ag(111), using interatomic potential from the Embedded Atom Method. For the hetero system Cu/Ag(111), this technique has uncovered mechanisms involving concerted motions such as shear, breathing and commensurate-incommensurate occupancies. Although the technique introduces complexities in storage and retrieval, it does not introduce noticeable extra computational cost.