Complex surface reconstructions solved by ab initio molecular dynamics

Complex surface reconstructions and surface oxides, in particular, often exhibit complicated atomic arrangements, which are difficult to resolve with traditional experimental methods, such as low energy electron diffraction (LEED), surface X-ray diffraction (SXRD) or scanning tunnelling microscopy (STM) alone. Therefore, ab initio density functional calculations are used as a supplement to the experimental techniques, but even then the structural determination usually relies on a simple trial and error procedure, in which conceivable models are first constructed and then tested for their stability in ab initio calculations. An exhaustive search of the configuration space is usually difficult and requires a significant human effort. Solutions to this problem, such as simulated annealing, have long been known, but are usually considered'to be too time-consuming in combination with first principles methods. In this work, we show that ab initio density functional codes are now sufficiently fast to perform extensive finite temperature molecular dynamics. The merits of this approach are exemplified for two cases, for a complex two-dimensional surface oxide on Pd(l 11), and for the oxygen induced c(6 x 2) reconstruction of V(110).