and the prediction of friction coefficients for such systems require very high accuracy in the calculated energies [@garvey:11a; @garvey:11b; @garvey:11c]. The Vienna Ab-Initio Simulation Package *VASP* [@kresse1993; @kresse1994a; @kresse1996a; @kresse1996b] applied to the copper system seems to be ideally suited for our study. The Projector Augmented Wave (PAW) code [@bloechl1994; @kresse:98] is much more accurate as for example embedded atom potentials but not as computationally expensive as highly accurate full-potential linearized augmented plane-wave (FLAPW) method which is needed sometimes for systems where the local bonding environment changes significantly [@garvey:11a]. We found that, while the considered system is small enough that the large number of calculations needed for the construction of accurate energy surfaces for various loads is still feasible, it is also realistic enough to work as a proof of concept for the proposed scheme to calculate friction forces on the nanoscale.

One of the main goals of this work is to examine the differences in nanofriction for different sliding directions, especially the differences between periodic and quasi aperiodic sliding paths. For all sliding directions we start in an energy minimum configuration corresponding to perfect fcc stacking. Different paths are distinguished by their angle with respect to the x-axis, see 