We present ab initio calculations of the short-range tip-surface interaction between K${}^{+}$- or Br${}^{-}$-terminated tips and a KBr(001) surface. Comparing the results for different tips represented by a small (KBr)${}_2$ or two larger (KBr)${}_6$ clusters, in most cases we find only relatively small differences in the tip-sample forces in the unrelaxed case. When relaxations of tip and surface are fully taken into account, we find a more pronounced dependence of the calculated forces on the coordination of the tip apex atom. This holds, in particular, for K${}^{+}$-terminated tips. We compare our results with experimental atomic force microscopy data. The latter do not reveal by themselves whether a K${}^{+}$ or a Br${}^{-}$ tip has been instrumental in a particular experiment. We find good agreement between our calculated two-dimensional force map for a K${}^{+}$-terminated tip and experiment. This result strongly supports the notion that the respective measurements have actually been carried out with a K${}^{+}$-terminated tip.

• Received 7 September 2011

DOI:https://doi.org/10.1103/PhysRevB.84.195402