## 3 Model Generation  
Based on previous work of the authors [24] and reference [18], three components were used in the control asphalt including the asphaltenes, aromatics, and saturates at a ratio of 5:27:41. The ratio of asphaltenes, aromatics and saturates was cited form the reference [18], and this ratio (asphaltenes, aromatics and saturates at 5:27:41) in the asphalt model was based on the asphaltene mass fraction (20.7 wt%) and alkane/aromatic carbon ratio (59.6 wt%, 19.7 wt%; wt% is weight percentage) [35]. The 1,7-Dimethylnaphthalene [36] (Figure 2a) and docosane [37] (Figure 2b) were used to represent the aromatics and saturates, respectively. 1,7-Dimethylnaphthalene (C\({}_{12}\)H\({}_{12}\)) was reported by [PERSON] [18] and [PERSON] [36] based on the ratio of alkane and aromatic in the asphalt. The docosane (C\({}_{22}\)H\({}_{46}\)) was reported by [PERSON] [18] and [PERSON] [37] based on the properties (melting and boiling points) of docosane and saturates. The asphaltene structure (Figure 2c) is from the references [18;38], C\({}_{64}\)H\({}_{52}\)S\({}_{2}\). In addition, the ESP was calculated and assigned to the components by NWChem. The control asphalt model was built through the compression of three components with NPT ensemble, and running time is around 1 ns. The start density of the control asphalt system is around 0.1 g/cm\({}^{3}\), and the target density is 1.0 g/cm\({}^{3}\), which is similar to that of the real asphalt. The low start density is good for relocations of atoms or components of systems during the energy optimization. Based on the previous property calculation of the asphalt model with the same components [18;24], the properties of the asphalt model are similar to those of the real asphalt tested in the laboratory. In this study, the ACEFF and ESP were assigned in the MD systems, and it is expected that the improvement of properties will be observed during the calculations. Therefore, the control asphalt model with ACEFF was generated to represent the control asphalt PG 58-28. In the laboratory tests, the modifier, xGNP graphene nanoplatelets, used in this study is produced by XG Sciences Inc., and its micro-image (Figure 2d) was obtained by the field emission scanning electron microscope (FE-SEM). The distance between the graphene layers is around 3.35 A, and the mole mass content of xGNP nanoplatelets in the modified asphalt is 2% by the weight of the control asphalt. During the preparation of xGNP modified asphalt in the laboratory, 2% xGNP multi-layer graphite particles were slowly added in the asphalt matrix at the temperature of 145 \({}^{\circ}\)C. The modified asphalt was sheared in the high shear machine for two hours to ensure that particles were well dispersed. Similarly, in the simulation test, the common multi-layer graphite model (Figure 2e) was used to represent the xGNP nanoplatelets, and 2% xGNP nanoplatelets by the weight of control model were randomly added to the control model. Mass mentioned in this study is based on 1 mole of the simulation box. The xGNP model with four layers was placed in the control asphalt model, and NPT ensemble was employed to compress the modified system. The xGNP modified asphalt model was generated and is shown in Figure 2f. The composition of the modified asphalt system is shown in Table 1. Different optimization methods mentioned above, the conjugated gradient method and the PPPM method, were adopted during the energy optimizations. The optimized system with the lowest energy was stable for calculations.