In computational chemistry and molecular dynamics, the combination rules or combining rules are equations that provide the interaction energy between two dissimilar non-bonded atoms, usually for the part of the potential representing the van der Waals interaction.
[2] The Lennard-Jones Potential is a mathematically simple model for the interaction between a pair of atoms or molecules.
[3][4] One of the most common forms is where ε is the depth of the potential well, σ is the finite distance at which the inter-particle potential is zero, r is the distance between the particles.
The potential reaches a minimum, of depth ε, when r = 21/6σ ≈ 1.122σ.
loosely reflect the radii of particle i and j respectively, their averages can be said to be the effective radii between the two particles at which point repulsive interactions become severe.
is related to the induced dipole interactions between two particles.
These rules are the most widely used and are the default in many molecular simulation packages, but are not without failings.
[22] Industrial equations of state have similar mixing and combining rules.
These include the van der Waals one-fluid mixing rules and the van der Waals combining rule, which introduces a binary interaction parameter
, There is also the Huron-Vidal mixing rule, and the more complex Wong-Sandler mixing rule, which equates excess Helmholtz free energy at infinite pressure between an equation of state and an activity coefficient model (and thus with liquid excess Gibbs free energy).