## 3 Construction and Simulation of Adsorption Models
### Simulation Parameter Settings and Model Configurations  
To explore the microscopic adsorption mechanism of anthracite under different pore conditions, the Sorption module was used in this adsorption simulation to simulate CH\({}_{4}\) adsorption under different pore diameters. The Analysis function in the Forcite module was employed to calculate the gas diffusion coefficient of the solid-gas saturated model. The specific steps are as follows [14; 15; 16]:  
1. In the Sorption module, the task type was set as \"Fixed pressure\", the method was \"Metropolis\", the simulation accuracy was \"Customized\", and the forcefield (Forcefield) was selected as the COMPASS II forcefield. For charges, the \"Forcefield assigned\" method was chosen. For the electrostatic interaction between atoms, the \"Atom Based\" method was selected, and for both van der Waals interaction and hydrogen bonding, the \"Atom Based\" method was also adopted. The number of equilibration steps was set to 1 \(\times\) 10\({}^{6}\), the number of production steps was set to 1 \(\times\) 10\({}^{6}\), and the number of escape steps was set to 20.
2. In the Sorption module, when the task type was selected as the \"Locate\" task, its corresponding parameter settings were the same as those of the \"Fixed pressure\" module.
3. In the Forcite module, the task type was set as \"Dynamics\", the temperature was chosen as 298 K, the NVT ensemble was selected, the simulation time was set to 500 ps, the time step length was set to 1.0 fs, and the temperature control method was selected as \"Nose\". The \"Analysis\" function was used to choose the diffusion coefficient (MSD).  
The surface and pores of the coal seam provide space for gas adsorption. Determining the surface area and pore volume of coal with different pore diameters is a prerequisite for exploring the influence of coal pore size on the CH\({}_{4}\) adsorption characteristics. The Build Layer tool was used to construct anthracite pore models with six different vacuum layer thicknesses ranging from 10 A to 60 A. The porosity was calculated under models with different vacuum layer thicknesses and the adsorption process was simulated. By using the Connolly algorithm and taking CO\({}_{2}\) molecules with a kinetic diameter of 3.3 A as molecular probes, the surface area, free volume, and occupied volume of the pore structures of coal with different pore diameters were determined, the porosity of coal samples with different pore diameters was calculated, and the influence of the change in porosity under  
Figure 1: Construction and optimization of anthracite macromolecule model.  
different pore diameter conditions on the adsorption performance was discussed [17]. The calculation formula is as follows:  
\[\phi=\frac{V_{f}}{V_{t}}\times 100\%\]  
In the formula, \(\phi\) represents the porosity, in percentage; \(V_{f}\) represents the free volume, in A\({}^{3}\); and \(V_{t}\) represents the total volume, in A\({}^{3}\).  
In Figure 2, the surface and pores of the coal are distinguished by blue and gray curved surfaces. Among them, the area surrounded by blue represents the free volume, and the area surrounded by gray represents the occupied volume. It can be seen from Figure 2 that as the pore diameter increases, the pore area at the adsorption sites in the coal pore structure increases significantly. To determine the influence of the pore size, the porosities of the macropores of coal with different pore diameters were calculated. The results of the porosities of anthracite under different pore diameter conditions are shown in Table 2. As the pore diameter gradually increases, the porosities of anthracite with different pore diameters increase to varying degrees.