Radial distribution function

2007-09-07T03:43:43Z

68.37.255.44: Small grammer edit

The '''radial distribution function''' is a special case of the [[pair distribution function]] for an isotropic system.
A [[Fourier analysis | Fourier transform]] of the radial distribution function results in the [[structure factor]], which is experimentally measurable.
==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)

:<math>\left. {\rm g}(r) \right. = e^{-\beta \Phi(r)} (1 + \rho {\rm g}_1 (r) + \rho^2 {\rm g}_2 (r) + ...)</math>

where <math>\rho</math> is the number of molecules per unit volume and <math>\Phi(r)</math>
is the [[intermolecular pair potential]]. The
function <math>{\rm g}(r)</math> is normalized to the value 1 for large distances.
As is known, <math>{\rm g}_1 (r)</math>, <math>{\rm g}_2 (r)</math>, ... can be expressed by
[[Cluster diagrams | 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

:<math>{\rm g}_1 (r_{12})= \int f (r_{13}) f(r_{23}) ~{\rm d}{\mathbf r}_3</math>

:<math>{\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})</math>

where <math>r_{ik}</math> is the distance <math>|{\mathbf r}_i -{\mathbf r}_k|</math>, where <math>f(r)</math>
is the [[Mayer f-function]]

:<math>\left. f(r) \right. = e^{-\beta \Phi(r)} -1</math>

and

:<math>\varphi (r_{12}) = \int f (r_{13}) f (r_{24}) f (r_{34}) ~ {\rm d}{\mathbf r}_3 {\rm d}{\mathbf r}_4</math>

:<math>\psi (r_{12}) = \int f (r_{13}) f (r_{23}) f (r_{24}) f (r_{34}) ~ {\rm d}{\mathbf r}_3 {\rm d}{\mathbf r}_4</math>

:<math>\chi (r_{12}) = \int f (r_{13}) f (r_{23}) f (r_{14}) f (r_{24}) f (r_{34}) ~ {\rm d}{\mathbf r}_3 {\rm d}{\mathbf r}_4</math>
==References==
#[http://dx.doi.org/10.1063/1.1723737 John G. Kirkwood and Elizabeth Monroe Boggs "The Radial Distribution Function in Liquids", Journal of Chemical Physics '''10''' pp. 394-402 (1942)]
#[http://dx.doi.org/10.1103/PhysRev.85.777 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)]
#[http://dx.doi.org/10.1063/1.1703948 J. L. Lebowitz and J. K. Percus "Asymptotic Behavior of the Radial Distribution Function", Journal of Mathematical Physics '''4''' pp. 248-254 (1963)]
[[category: statistical mechanics]]

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Radial distribution function