# Effect of Aggregate Crystalline Surface Anisotropy on
## 2 Molecular Models and Simulation Methods  
### Molecular Model of Asphalt  
Asphalt is a multi-scale chemical mixture with a complex composition that includes various elements, making it challenging to accurately represent its true chemical structure. Common methods for the construction of asphalt models include the average molecular model and the component molecular model. While the average molecular model can reflect the structural differences between different asphalts, interactions between different molecular structures within the asphalt are not effectively captured. [PERSON] and [PERSON] proposed a 12-component model for SHRP AAA-1, AAK-1, and AAM-1 asphalt binders, which comprises the four major fractions of constituent molecules [18]. The four components--resin, saturates, aromatics, and asphaltenes--are each represented by a variety of molecules, allowing for the more accurate simulation of asphalt's structure. This model is widely used in MD simulations related to asphalt. In this study, the 12-component AAA-1 asphalt model was directly adopted. The molecular structure of asphalt 12 was constructed as shown in Figure 1, with the molecular parameters of each component displayed in Table 2.  
After constructing the 12-component asphalt model, the _Forcite_ module was employed to perform annealing simulations and geometry optimization to achieve an equilibrium model with a stable volume and energy. Both the initial asphalt model and the optimized asphalt model are shown in Figure 2.  
The annealed asphalt model underwent a dynamic equilibrium process in a 1000 ps NPT system at 298 K. As shown in Figure 3, the density curve initially increased continuously with the simulation time; it then fluctuated within a certain range and was stabilized at a density of 1.011 g/cm\({}^{3}\). The actual density of asphalt material at 298 K is approximately 1.000 g/cm\({}^{3}\)[19], indicating that the simulated density closely matched the actual value. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Components**} & \multirow{2}{*}{**Molecular Name**} & **No. of Molecules** & **Molecular** & **Molecular Mass** & **Mass Ratio** \\  & & **in Proportion** & **Formula** & **(g/mol)** & **(\%)** \\ \hline \multirow{6}{*}{Resin (Res)} & Pyridinobpane & 4 & C\({}_{36}\)H\({}_{57}\)N & 503.9 & 6.2 \\  & Thio-isorenieratane & 4 & C\({}_{40}\)H\({}_{60}\)S & 572.9 & 7.0 \\  & Trimethylbenzene-oxane & 5 & C\({}_{29}\)H\({}_{50}\)O & 414.7 & 6.4 \\  & Quinolinobpane & 4 & C\({}_{40}\)H\({}_{59}\)N & 554.0 & 6.8 \\  & Benzobisbenzothiophene & 15 & C\({}_{18}\)H\({}_{10}\)S\({}_{2}\) & 290.4 & 13.4 \\ \hline Saturate (Sat) & Squalane & 4 & C\({}_{30}\)H\({}_{62}\) & 422.9 & 5.2 \\  & Hopane & 4 & C\({}_{35}\)H\({}_{62}\) & 482.8 & 5.8 \\ \hline \multirow{2}{*}{Aromatics (Aro)} & PHPN & 11 & C\({}_{35}\)H\({}_{44}\) & 464.8 & 15.7 \\  & DOCHN & 13 & C\({}_{30}\)H\({}_{46}\) & 406.8 & 16.2 \\ \hline \multirow{2}{*}{Asphaltenes (Asp)} & Pyrrole & 2 & C\({}_{66}\)H\({}_{81}\)N & 888.5 & 5.5 \\  & Phenol & 3 & C\({}_{42}\)H\({}_{54}\)O & 575 & 5.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed molecular information of the 12-component asphalt model. Figure 1: The 12-component asphalt model. Resin: (**a**) Pyridinohopane, (**b**) Thio-isorenieratane, (**c**) Trimethylbenzene-oxane, (**d**) Quinolinohopane, (**e**) Benzobisbenzothiophene. Saturates: (**f**) Squalane, (**g**) Hopane. Aromatics: (**h**) PHPN, (**i**) DOCHN. Asphaltenes: (**j**) Pyrrole, (**k**) Phenol, (**l**) Thiophene. (We use gray to indicate carbon, white to indicate hydrogen, red to indicate oxygen, blue to indicate nitrogen, and yellow to indicate sulfur). The temperature at which asphalt transitions from a viscous flow state to a glassy state is known as the glass transition temperature (\(Tg\)). \(Tg\) is a critical parameter affecting the physical and mechanical properties of asphalt.