Virial coefficients B i appear as coefficients in the virial expansion of the pressure of a many-particle system in powers of the density, providing systematic corrections to the ideal gas law. They are characteristic of the interaction potential between the particles and in general depend on the temperature. The second virial coefficient B 2 depends only on the pair interaction between the particles, the third ( B 3 ) depends on 2- and non-additive 3-body interactions, and so on.