The `dsmc` repository provides foundational code for the Direct Simulation Monte Carlo (DSMC) method, a powerful computational technique used to simulate rarefied gas flows. Unlike traditional continuum fluid dynamics models such as Navier-Stokes equations, DSMC is particularly effective in environments where the mean free path of molecules is comparable to or larger than the characteristic physical length scale, making it indispensable for high-altitude aerodynamics, vacuum technology, and micro-scale fluid dynamics. It directly simulates the behavior of individual gas molecules, making it suitable for non-equilibrium flow conditions. This collection primarily features original FORTRAN implementations of the DSMC method developed by Graeme A. Bird, a pioneer in the field. The method operates by tracking a statistically representative number of simulated particles, which move and interact probabilistically. Key technical aspects include the use of probabilistic collision models, such as the Variable Hard Sphere (VHS) and Variable Soft Sphere (VSS) models, to govern intermolecular and molecule-surface interactions. The code is designed to handle complex geometries, large pressure gradients, and inherently unsteady flow characteristics. While the core repository contains FORTRAN code, the DSMC method itself has seen implementations in various languages like C++, Java, and Python, often optimized for parallel processing on high-performance computing (HPC) clusters and GPUs to manage the computational intensity of particle-based simulations. The `dsmc` code and the DSMC method are widely applied across numerous scientific and engineering disciplines. In aerospace, it is used for analyzing re-entry aerodynamics, spacecraft-plume interactions, and gas flows around vehicles in thin atmospheres. Microelectronics benefits from DSMC for modeling gas behavior in vacuum chambers during semiconductor manufacturing processes. Other applications include the study of microelectromechanical systems (MEMS), vacuum pump design, and various PVD (Physical Vapor Deposition) coating processes. Its ability to accurately capture non-equilibrium effects and high gradients makes it crucial for understanding phenomena where continuum assumptions break down. The `schuberm/dsmc` GitHub repository serves as a valuable resource for researchers by providing access to the classic Bird's DSMC codes (e.g., DSMC1.FOR, DSMC1U.FOR, DSMC2A.FOR), which are frequently referenced in scientific literature. The repository's purpose is to offer these foundational implementations, allowing users to study, adapt, and build upon the original algorithms. While the repository itself has a basic README, the comprehensive documentation for the underlying DSMC method is extensively available in numerous scientific papers, textbooks, and academic resources that detail Bird's work. Users will need a FORTRAN compiler to utilize these specific codes. Modern DSMC projects often integrate with parallel computing libraries like MPI or OpenMP for distributed simulations.