Radial distribution function

Density Expansion of the radial distribution function

The radial distribution function of a compressed gas may be expanded in powers of the density (Ref. 2)

$\left.{\rm {g}}(r)\right.=e^{-\beta \Phi (r)}(1+\rho {\rm {g}}_{1}(r)+\rho ^{2}{\rm {g}}_{2}(r)+...)$

where $\rho$ is the number of molecules per unit volume and $\Phi (r)$ is the intermolecular pair potential. The function ${\rm {g}}(r)$ is normalized to the value 1 for large distances. As is known, ${\rm {g}}_{1}(r)$, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\rm g}_2 (r)} , ... can be expressed by cluster integrals in which the position of of two particles is kept fixed. In classical mechanics, and on the assumption of additivity of intermolecular forces, one has

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\rm g}_1 (r_{12})= \int f (r_{13}) f(r_{23}) ~{\rm d}r_3}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {\rm g}_2 (r_{12})= \frac{1}{2}({\rm g}_1 (r_{12}))^2 + \varphi (r_{12}) + 2\psi (r_{12}) + \frac{1}{2} \chi (r_{12})}

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle r_{ik}} is the distance Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |r_i -r_k|} , where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(r)} is the Mayer f-function

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left. f(r) \right. = e^{-\beta \Phi(r)} -1}

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \varphi (r_{12}) = \int f (r_{13}) f (r_{24}) f (r_{34}) ~ {\rm d}r_3 {\rm d}r_4}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \psi (r_{12}) = \int f (r_{13}) f (r_{23}) f (r_{24}) f (r_{34}) ~ {\rm d}r_3 {\rm d}r_4}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \chi (r_{12}) = \int f (r_{13}) f (r_{23}) f (r_{14}) f (r_{24}) f (r_{34}) ~ {\rm d}r_3 {\rm d}r_4}

References

1. John G. Kirkwood and Elizabeth Monroe Boggs "The Radial Distribution Function in Liquids", Journal of Chemical Physics 10 pp. 394-402 (1942)
2. B. R. A. Nijboer and L. Van Hove "Radial Distribution Function of a Gas of Hard Spheres and the Superposition Approximation", Physical Review 85 pp. 777 - 783 (1952)