Mayer f-function

Summary

The Mayer $f$-function of a pair potential $\varphi$ at inverse temperature $\beta$ is $f(x) = e^{-\beta \varphi(x)} - 1$. It turns the Boltzmann factor of a pair of particles into a quantity that vanishes when the particles are far apart. Expanding products of factors $1 + f$ gives sums over graphs. This is the starting point of the cluster expansion and of the virial expansion of a real gas.