**2. Geometry and boundary conditions**

The structures considered are single-crystal face centered cubic (fcc) Cu beams of length *L* = 100*a*<sup>0</sup> and square cross section with three different side lengths *s* = 6*Na*<sup>0</sup> , *N* = 1, 2, 3, with *a*<sup>0</sup> = 3.615 Å denoting the lattice parameter for Cu. Each beam holds a symmetrically placed, square-shaped through-the-thickness void of width *w* and height *h*, cf. **Figure 1(a)** where a centrally placed coordinate system (*x, y, z*) is introduced. The sensitivity to voids will be investigated by varying the width *w*, keeping the relative void height *h* = *s*/3 constant.

Two different crystallographic orientations are considered to determine its influence on the mechanical response. For the first orientation, referred to as the [100]-orientation, the coordinates (*x, y, z*) coincide with crystallographic directions according to: *x =* [100], *y* = [010] and *z* = [001], cf. **Figure 1(b)**. For the second, referred to as the [110]-orientation, *x* = [110], *y* = [−110] and *z* = [001], cf. **Figure 1(c)**.

A beam is built from the repetition of fcc Cu unit cells, and to mimic clamped end boundary conditions, all atoms within four unit cells at each end of the beam are restricted from movements in the *y*- and *z*-directions. During the relaxation step, one atom is fixed in all directions.

**Figure 1.** (a) Beam configuration and coordinate system. (b) and (c) Crystallographic orientations.

The atoms in between the clamped ends are free to move in all directions without constraints. The beams are loaded in tension along the *x*-axis until final rupture. The load is displacement controlled through applying a constant velocity, *vend*, in the +*x*- and –*x*-directions to all atoms within the clamped beam ends.
