for a given sliding path are counted as positive.

As already mentioned all energies and forces are calculated with the DFT package *VASP* using the PAW method with an energy cut-off of . For the exchange and correlation potential, the Generalized Gradient Approximation (GGA) in the version devised by Perdew, Burke and Ernzerhof (PBE) is applied [@perdew:97]. As the strong chemical bonds in copper are well described with PBE and our results rely on the calculation of total energies, there is no need to explicitly consider van der Waals forces or meta-GGA. The Brillouin zone sampling is performed on a $\Gamma$-centered $24\times24\times1$ k-grid ensuring a total energy convergence better than per simulation cell. Atomic relaxations were converged to in total energy, with each electronic calculation being converged to .

![(color online) Energy landscape of a constant load scan for two fcc Cu(111) slabs displaced relativ to each other. In addition, two periodic sliding paths with $0^\circ$ (red) and $30^\circ$ (blue) are shown. The angles are defined with respect to the Cartesian x-direction. The arrows $a_1$ and $a_2$ are two basis vectors of the rhombohedral unit cell. The x-direction dissects the plane spanned by $a_1$ and $a_2$ and points into the 