Simulation of surface processes

Abstract

Computer simulations of surface processes can reveal unexpected insight regarding atomic-scale structure and transitions. Here, the strengths and weaknesses of some commonly used approaches are reviewed as well as promising avenues for improvements. The electronic degrees of freedom are usually described by gradientdependent functionals within Kohn-Sham density functional theory. Although this level of theory has been remarkably successful in numerous studies, several important problems require a more accurate theoretical description. It is important to develop new tools to make it possible to study, for example, localized defect states and band gaps in large and complex systems. Preliminary results presented here show that orbital density-dependent functionals provide a promising avenue, but they require the development of new numerical methods and substantial changes to codes designed for Kohn-Sham density functional theory. The nuclear degrees of freedom can, in most cases, be described by the classical equations of motion; however, they still pose a significant challenge, because the time scale of interesting transitions, which typically involve substantial free energy barriers, is much longer than the time scale of vibrations-often 10 orders of magnitude. Therefore, simulation of diffusion, structural annealing, and chemical reactions cannot be achieved with direct simulation of the classical dynamics. Alternative approaches are needed. One such approach is transition state theory as implemented in the adaptive kinetic Monte Carlo algorithm, which, thus far, has relied on the harmonic approximation but could be extended and made applicable to systems with rougher energy landscape and transitions through quantum mechanical tunneling.

Transition state theory approach to polymer escape from a one dimensional potential well

“…One way to approach such multiple local minima problems is systematic coarse graining [29]. Alternatively, the full TST can be applied as opposed to HTST where, for example, the reversible work formulation is used to identify an optimal transition state and estimate the free energy change in going from the initial state to the transition state [30,31,32].…”

Method for finding mechanism and activation energy of magnetic transitions, applied to skyrmion and antivortex annihilation

“…The NEB method has turned out to be a powerful tool for determining transition mechanisms of atomic rearrangements, such as chemical reactions and diffusion events. Complex and counterintuitive mechanisms have, for example, been found for catalytic reactions and diffusion events controlling the morphology of growing crystal surfaces [22,23].…”

Surface chemistry: Key to control and advance myriad technologies

“…The contribution here by Jonsson (28) summarizes some of the important new directions that are being pursued to achieve these goals. That paper also highlights recent improvements in calculating dynamical effects, such as accurately finding transition states, computing preexponential factors, and predicting tunneling rates (which Jonsson (28) shows to only be important below ∼300 K). The description of transition states plays a dominant role in understanding the all important rates of catalytic reaction steps.…”