3.5.4. Construction of the Coal Molecular Structure

Based on the number of atoms per element, the molecular formula of gas coal was determined to be C181H150O9N3. The molecular structure diagram of gas coal was constructed by the Chemdraw software. The MestReNova software was used to calculate the spectrum of the coal molecular structure. Then, the calculated spectrum was compared with the 13C-NMR experimental spectrum, and the comparison chart was further adjusted and optimized according to XRD data. The obtained structural model is shown in Figure 5, and the comparison results of the 13C-NMR spectra are shown in Figure 6.

**Figure 5.** Molecular structure model of gas coal.

**Figure 6.** Experimental and calculated 13C-NMR spectra.

According to Figure 6, the calculated spectrum agreed well with the experimental spectrum, which indicated that the molecular model was constructed accurately. However, the largest relative deviation was obtained in the oxygen−carbon region with a chemical shift from 170 ppm to 220 ppm. This finding is mainly due to the side-band effect of the oxygen−carbon region in the experimental process, which led to errors of large intensity and high peak values in the oxygen−carbon region of the experimental spectrum [40]. Therefore, the oxygen−carbon region was further investigated by verifying the rationality of the molecular model through 13C-NMR carbon spectroscopy.

#### **4. Optimization and Validation of the Coal Macromolecule Model**

The coal macromolecular structure model was only a collection of analytical results of experimental data, which must be simulated for further analysis and validation. The Materials Studio (MS) software was used to optimize the structure model of coal macromolecules, using molecular mechanics (MM) and molecular dynamics (MD) methods to simulate the structural properties and interactions between molecules. In molecular or periodic system simulations, the Focite module could solve the energy calculation, geometric optimization, and the process of dynamic simulation. The force field was Dreiding, and the parameters of the Dreiding force field were obtained by fitting the quantum chemistry calculation data, and the use of the Dreiding force field reflected the application of quantum chemical theory in molecular simulations.

The information and density of each carbon atom in coal is an important parameter to reflect the molecular structure of coal. The rationality of the model was verified by comparing the agreement between the calculated and experimental carbon spectra, as well as the similarity between the experimental and simulated densities. The refined coal macromolecule model was also reasonably confirmed, because the calculated spectrum was in satisfactory agreement with the experimental spectrum. Therefore, it was not discussed again in this section.

#### *4.1. Optimal Structure*

Molecules exist mainly in the lowest energy form in natural situations. Thus, only the lowest energy model represented the optimal state of the molecular structure under study. The lowest energy could not be guaranteed for the built model during the modeling process. Thus, it was necessary to optimize and find the lowest energy model to ensure that the subsequent results were meaningful [18].

#### 4.1.1. Optimized Method and Parameter Setting

The molecular structure model of gas coal was imported into MS software and automatically hydrogenated to fullness. The lowest energy model was obtained by MM and MD calculations. The module selection and parameter settings were based on the simulation calculation method in the literature [40]. MM and MD calculations were conducted in the Forcite module in MS. The charge distributions were obtained through the charge equilibrium method in the MM calculation. The MM parameters were as follows: the computational method was a smart minimizer, the maximum number of iterations steps was set to 5000 steps considering the computational accuracy and convergence time cost, and the convergence criterion was medium. The Dreiding [41,42] force field was selected, because it was suitable for calculating the structures and various properties of most types of molecules and materials with high accuracy. A fixed-volume, fixed-temperature (NVT) MD simulation was conducted. The MD parameters were as follows: the temperature was 300–600 K, the heating order was 5, the heating rate was 60 K/time. The temperature control program selected Nose [43] to ensure that the distribution of system degrees of freedom was normative, and the time step was 0.1 fs.
