# A Coarse-Grained Molecular Model for Simulating Self-Healing of Bitumen
## 2 Establishment of Molecular Model  
### All-Atom Molecular Model (MD)  
Our coarse-grained molecular model is derived from the all-atom MD simulation. In the all-atom MD process, we used the four-components with twelve molecules model of bitumen (Table 1) proposed by [PERSON] and [PERSON] [20], who characterized AAA-1, AAK-1 and AAM-1 studied in the Strategic Highway Research Program (SHRP) by varying the ratio of the number of each type of molecule. This model contains four components: asphaltene, aromatics, saturate and polar aromatic, with N, O and S introduced, which gives a more complete picture of the diversity and complexity of bitumen than the earlier three-component model containing only C and H.  
The force field is the basis of MD simulation which, as an approximate treatment of quantum mechanical methods, ignore computationally resource-intensive electronic behavior and can be used for molecular systems with a large number of atoms. There are several suitable force fields for organic matter available for MD simulations of bitumen, such as Universal, Compass, Compass II, OPLS-aa and Gromos96, etc. In our all-atom simulations, the Compass II force field is chosen, which is a molecular force field for studying condensed matter optimization from the atomic level to simulate both organic molecular systems as well as inorganic ones, it is suitable for the simulations of bitumen. All simulations in this paper were completed using Materials Studio 2017. The all-atom molecular model of bitumen is simulated based on the following steps:  
1. Twelve different bitumen molecules are drawn and geometrically optimized to eliminate errors in bond lengths, bond angles, etc. ;
2. Molecules are placed into a computational box with periodic boundary conditions with an initial density of 0.1 g/cm\({}^{3}\);
3. The system is relaxed for 300 ps in the NVT ensemble, the temperature is set to 298 K. The NVT ensemble refers to a constant temperature, constant volume ensemble where, through NVT relaxation, the bitumen molecules will fully move in the calculation box, thus mixing uniformly and reaching a relatively low energy position, with no change in density during this process;
4. It is annealed five times in the temperature range of 263-433 K, which corresponds to the working temperature of road bitumen. Annealing means that the temperature of the system is repeatedly raised and lowered, allowing some of the molecules to break through the energy barrier to reach a less energetic and more stable structure upon return to normal temperature;
5. It is simulated for 300 ps in the NPT ensemble, T = 298 K, P = 1 atm. The NPT ensemble refers to a constant temperature and pressure ensemble where the total energy and system volume can be freely varied, and the density of the bitumen system will stabilize through NPT relaxation. For the control of the NVT and NPT ensembles described above, temperature control and pressure regulation operations are required. We adopt the Berendsen method [21; 22], the basic idea of which is to couple the system to a constant temperature external heat bath and to regulate the temperature and pressure of the system by absorbing or releasing energy from the heat bath. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Molecular Name** & **Chemical Formula** &
\begin{tabular}{c} **Number of Molecules** \\ **Contained in the System** \\ \end{tabular} \\ \hline Asphaltene A & C\({}_{66}\)H\({}_{81}\)N & 3 \\ Asphaltene B & C\({}_{42}\)H\({}_{54}\)O & 2 \\ Asphaltene C & C\({}_{51}\)H\({}_{62}\)S & 3 \\ Saturate A & C\({}_{30}\)H\({}_{62}\) & 4 \\ Saturate B & C\({}_{35}\)H\({}_{62}\) & 4 \\ Aromatic A & C\({}_{35}\)H\({}_{44}\) & 11 \\ Aromatic B & C\({}_{30}\)H\({}_{46}\) & 13 \\ Polar aromatic A & C\({}_{40}\)H\({}_{59}\)N & 4 \\ Polar aromatic B & C\({}_{35}\)H\({}_{57}\)N & 4 \\ Polar aromatic C & C\({}_{18}\)H\({}_{10}\)S\({}_{2}\) & 15 \\ Polar aromatic D & C\({}_{40}\)H\({}_{60}\)S & 4 \\ Polar aromatic E & C\({}_{29}\)H\({}_{50}\)O & 5 \\ \hline \hline \end{tabular}
\end{table}