Pair distribution function

For a fluid of $N$ particles, enclosed in a volume $V$ at a given temperature $T$ (canonical ensemble) interacting via the `central' intermolecular pair potential $\Phi (r)$, the two particle distribution function is defined as

${\rm {g}}_{N}^{(2)}({\mathbf {r} }_{1},{\mathbf {r} }_{2})=V^{2}{\frac {\int ...\int e^{-\beta \Phi ({\mathbf {r} }_{1},...,{\mathbf {r} }_{N})}{\rm {d}}{\mathbf {r} }_{3}...{\rm {d}}{\mathbf {r} }_{N}}{\int e^{-\beta \Phi ({\mathbf {r} }_{1},...,{\mathbf {r} }_{N})}{\rm {d}}{\mathbf {r} }_{1}...{\rm {d}}{\mathbf {r} }_{N}}}$

where $\beta =1/(k_{B}T)$, where $k_{B}$ is the Boltzmann constant.

Exact convolution equation for $g(r)$

See Eq. 5.10 of Ref. 1:

$\ln g(r_{12})+{\frac {\Phi (r_{12})}{kT}}-E(r_{12})=n\int \left(g(r_{13})-1-\ln g(r_{13})-{\frac {\Phi (r_{13})}{kT}}-E(r_{13})\right)(g(r_{23})-1)~{\rm {d}}{\mathbf {r} }_{3}$

where, i.e. $r_{12}=|{\mathbf {r} }_{2}-{\mathbf {r} }_{1}|$.

See also

• Radial distribution function
• Compressibility equation
• Pressure equation
• Energy equation

References

1. J. S. Rowlinson "The equation of state of dense systems", Reports on Progress in Physics 28 pp. 169-199 (1965)