## 3 Results
### Hydrostatic Compression  
As mentioned in Section 2, the composite precursor was hydrostatically compressed to obtain zero stresses in the system. Then, it was hydrostatically compressed at 1000 K to obtain a composite with maximal densities of 5.69 g/cm\({}^{3}\) (BOP), 5.59 (EAM-Morse) and 5.64 (Morse). In Figure 2a, snapshots of the composite precursor, obtained with three different potential functions, are presented after exposure at 300 K, followed by hydrostatic compression to the state with zero stress. In Figure 2b,c, pressure (b) and potential energy (c) as a function of compression strain are presented during hydrostatic compression at 1000 K. In Figure 2d, snapshots of the composite obtained with three different potential functions are presented. Figure 2: (**a**) Snapshots of the composite precursors obtained with different potentials after exposure at 300 K followed by hydrostatic compression. (**b,c**) Pressure and potential energy changes during hydrostatic compression. (**d**) Snapshots of the composite, obtained with different potentials. Cu atoms are shown in orange and C atoms are shown in black. As is shown in Figure 2a, after pre-compression, in the structure obtained with the EAM-Morse and Morse potentials, the Cu nanoparticles coagulate. For structure \(\sigma\) obtained with the BOP potential, the Cu nanoparticles are covered with GFs, which occurred during exposure and was enhanced during pre-compression. Thus, we have something like graphene \"balls\" with nanoparticles inside. For all three cases, it was found that, with increase in strain, the pressure increased rapidly (see Figure 2b), which is common for graphene networks under compression [34]. However, for CG-Cu\({}_{BOP}\), the maximum stress was 650 GPa, while for CG-Cu\({}_{EM}\) and CG-Cu\({}_{M}\), it was almost two times lower. This difference can be explained by the fact that the BOP potential somehow makes both the graphene and the nanoparticles more rigid and, therefore, greater pressure is required to compress the structure. By comparison of Figure 2b,c, it can be seen that, before \(\varepsilon=0.08\) (BOP) and \(\varepsilon=0.04\) (EAM-Morse, Morse), the potential energy is almost unchanged and the stress is close to zero. Before these values of strain, there are still many pores in the composite precursor and the neighboring elements interact weakly with each other. After \(\varepsilon=0.08\) (\(\varepsilon=0.04\)), the GF begins to interact and new covalent bonds appear between the GFs. Thus, the separated graphene elements transform into a graphene network. The curves for the structure obtained with the EAM-Morse and Morse potentials are very close, both for \(\sigma(\varepsilon)\) and \(E_{pot}(\varepsilon)\). The main difference is that, for the structure simulated with BOP, the potential energy increases, while for the EAM-Morse and Morse potentials, it decreases. This can be explained as a result of the BOP potential causing both the Cu nanoparticles and graphene flakes to become more rigid. Consequently, in the process of hydrostatic compression, due to the high rigidity of the nanoparticles and graphene, the potential energy in the system increases. For use of the other two potentials, the copper nanoparticles, in this case, are softer; in addition, the AIREBO potential enables graphene to bend more easily and, thus, hydrostatic compression occurs much more easily. This is also connected with the simulation process itself. It should be noted that compression with the EAM-Morse and Morse potentials stopped automatically when the structure compressed to a quite high density and physically could not be compressed further. However, for compression with BOP, the structure could be compressed to an unrealistic density and stopped manually when an average density \(5.6\,\mathrm{g/cm^{3}}\) was achieved. This was also confirmed by the constant increase in the potential energy during compression.