# Nano-Interaction Mechanism Between Crumb Rubber and Asphal Components: A Molecular Dynamic Study
## 2 Modeling and Simulation Method  
### Asphalt Modeling  
#### 2.1.1 Base Asphalt Model  
Comparing and analyzing the existing molecular models of asphalt [25] and considering the computational accuracy and computational efficiency, the four-component twelve-molecular model proposed by [PERSON] et al. was used for base asphalt model construction [26; 27]. The model construction mechanism was based on quantum mechanics and density functional theory (DFT). It was proposed for the analysis of asphalt in the U.S. SHRP program. The analytical values of its physical and chemical reactions agreed with the measured results.  
The model was based on the classical four-component asphalt model of saturates, aromatics, resins, and asphaltene (Figure 1). According to the differences in the composition and content of each component, 12 molecules, as in Table 1, were selected for model construction [13; 28].  
Figure 1: Asphaltene-2 and Asphaltene-3  
After constructing the four-component 12-molecule model, the Forcite module was used to set the forcefield and charge of each molecule. After checking the accuracy of the molecular structure, the Forcite module was used for geometry optimization to ensure the rationality of the structural arrangement of the assembled molecules. In Geometry Optimization, the optimization algorithm adopts the \"Smart\" mode. The maximum number of iterations was taken as 20,000. All computation steps are 1 fs.  
The asphalt cell box was constructed according to the number of molecules calculated in Table 1. The target density of the crystal cell was set to 1.0 g/cm\({}^{3}\). The COMPASS II force field was used [27]. In addition, to ensure the random distribution of the structure of each component during the model construction process and to avoid the phenomena of entanglement and overlap, the initial density was set to 0.2 g/cm\({}^{3}\). It can transition from the initial density to the target density during the formation of the cell. The established asphalt model was geometrically optimized to reduce the model energy and ensure its structural stability.  
The annealing procedure was measured to optimize the structure further and eliminate the irrational structure and bad contact between the molecule structures. The initial temperature was set at 298 K and the termination temperature at 1000 K. A total of 10 annealing processes were carried out. After completing the annealing treatment, the energy changed after each annealing and structure optimization cycle were analyzed. The lowest energy frame corresponding to the asphalt model structure (Figure 2) was taken for subsequent analysis.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Components** & **Molecules** & **Molecular** & **Relative Molecular** & **Mass Fraction/\%** \\  & **Number** & **Formula** & **lar Mass/(g/mol)** & **Mass Fraction/\%** \\ \hline \multirow{3}{*}{Saturates} & 4 & C\({}_{30}\)H\({}_{62}\) & 422.9 & 5.2 & \\  & 4 & C\({}_{35}\)H\({}_{62}\) & 482.8 & 5.8 & \\ \multirow{3}{*}{Aromatics} & 11 & C\({}_{35}\)H\({}_{44}\) & 464.8 & 15.7 & \\  & 13 & C\({}_{30}\)H\({}_{46}\) & 406.8 & 16.2 & \\ \multirow{3}{*}{Resins} & 4 & C\({}_{36}\)H\({}_{57}\)N & 503.9 & 6.2 & \\  & 4 & C\({}_{40}\)H\({}_{60}\)S & 572.9 & 7.0 & \\ \multirow{3}{*}{Resins} & 5 & C\({}_{29}\)H\({}_{50}\)O & 414.7 & 6.4 & 39.8 \\  & 4 & C\({}_{40}\)H\({}_{59}\)N & 554.0 & 6.8 & \\ \multirow{3}{*}{Asphaltene} & 15 & C\({}_{18}\)H\({}_{10}\)S\({}_{2}\) & 290.4 & 13.4 & \\  & 3 & C\({}_{42}\)H\({}_{54}\)O & 575 & 5.3 & \\ \cline{1-1}  & 2 & C\({}_{66}\)H\({}_{81}\)N & 888.5 & 5.5 & \\ \cline{1-1}  & 3 & C\({}_{51}\)H\({}_{62}\)S & 707.2 & 6.5 & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Molecular structure parameters of base asphalt four-component 12.  
Figure 2: Asphalt model with lowest energy after annealing cycle.