ShortrangeRepulsion

ShortrangeRepulsion is a general term describing the strongly repulsive component of interparticle interactions that dominates when particles are brought into close proximity. It acts to prevent overlap due to quantum mechanical restrictions and electrostatic repulsion, and is central to the concept of excluded volume in dense matter. In classical pair potential models, short-range repulsion is modeled as a potential that either diverges or becomes very large as the separation r approaches a characteristic contact distance, often denoted sigma. Common realizations include the hard-sphere potential, which is infinite for r<sigma and zero for r>=sigma, and the repulsive part of the Lennard-Jones potential, which scales approximately as r^-12 at short distances. In modern simulations, the Weeks-Chandler-Andersen (WCA) potential isolates the purely repulsive part of the LJ interaction, truncating and shifting at its minimum to avoid attractive contributions. Soft-core alternatives use finite but steep repulsion, such as exponential or inverse-power forms, to approximate the same physics with differentiable forces.

The short-range repulsion determines the structure and equation of state of dense fluids by setting the effective particle size and contact constraints, influencing the first peak of the radial distribution function and transport properties. It is typically complemented by attractive forces at longer range to reproduce real material behavior. In practice, choosing a repulsive form involves trade-offs between accuracy and computational efficiency, and parameters are often tuned to match experimental data or higher-level theories. See also: Lennard-Jones potential, hard-sphere model, WCA potential.