MgO addimer diffusion on MgO(100): A comparison of<i>ab initio</i>and empirical models

Abstract

Diffusion of a MgO dimer on a MgO(100) surface is investigated using both density functional theory (DFT) and empirical ionic potentials. Barriers for diffusion via hop and exchange mechanisms are calculated. A qualitative difference is found between DFT and the empirical potential for the oxide exchange barrier. DFT predicts a saddle point for the process with a barrier of $0.88\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, whereas the empirical potential of Lewis and Catlow, with a formal charge of $\ifmmode\pm\else\textpm\fi{}2.0e$, finds this structure to be a stable intermediate minimum with an energy of $0.19\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, relative to the most stable addimer structure. The empirical potential predicts that the oxide hop and exchange mechanisms are equally likely; whereas, DFT shows that the oxide adion hop mechanism has a lower energy barrier. A Bader population analysis of the DFT charge density indicates that the magnesium and oxide ions have partial charges of magnitude $\ifmmode\pm\else\textpm\fi{}1.7e$. Using an empirical potential with this partial charge, the local minimum in the oxygen exchange process becomes a saddle at $0.62\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which is in better agreement with DFT. The standard deviation between the energy of the DFT minima and the saddle points with those of the empirical potential was reduced from $0.32\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ when using the formal charge parameters of Lewis and Catlow to $0.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ using partial charges. The qualitative agreement found for each diffusion barrier using the partial charge model suggests that a Bader analysis can be used to obtain suitable partial charges for constructing empirical potentials.