Please see the following papers for details about the DSMC model:
Kinetic approaches to modeling the cometary coma recognize that in much of the coma, collisions occur between molecules, but not enough to maintain thermal equilibrium in large and sometimes critical regions. Fully kinetic models based on the Direct Simulation Monte Carlo (DSMC) method have been applied to the cometary coma as a practical method to solving the general collisional Boltzmann equation. DSMC was developed to simulate the transition regime, where the mean free path of molecules is too large for continuum hydrodynamics to be applicable. Therefore, gas molecules are modeled as a sample of individual particles and simulated as they move around within a grid, colliding with other particles and with solid objects (if any). Macroscopic properties, such as density, velocity and temperature are computed by performing the standard kinetic theory averaging of particle masses, locations, velocities, and internal energies over the particle distribution function. DSMC is based on the rarefied-gas assumption that over a short time interval or 'step' the molecular motion and the intermolecular collisions are uncoupled and therefore can be calculated independently. Molecules are moved over the distances appropriate for this time step, followed by the calculation of a representative set of collisions. The time step is small compared to the mean collision time, and the results are independent of its actual value. DSMC will produce a hydrodynamic-like solution in the collisional fluid limit.
We have developed a gas kinetic dusty-gas DSMC comet model code. It is capable of being run for 1D-spherical, 2D axisymmetric and fully 3D problems. The photochemistry and photochemical kinetics for the following parent molecules are now available: H2O, CO2, CO, CH3OH, H2CO, C2H6, C2H4, C2H2, HCN, NH3, and CH4. The model subsequently tracks the following dissociation products: H, H2, O, OH, C, CH, CH2, CH3, N, NH, NH2, C2, C2H, C2H5, CN, and HCO. Refractory dust particles are included also as many simulation particles where the particle density and size distribution can be specified.
The fundamental descriptions and various applications of our DSMC comet model are found the papers listed above.