Ornstein-Zernike relation from the grand canonical distribution function

Defining the local activity by

$\left.z(r)\right.=z\exp[-\beta \psi (r)]$

where $\beta =1/k_{B}T$, and $k_{B}$ is the Boltzmann constant. Using those definitions the grand canonical partition function can be written as

$\Xi =\sum _{N}^{\infty }{1 \over N!}\int \dots \int \prod _{i}^{N}z(r_{i})\exp(-\beta U_{N})dr_{1}\dots dr_{N}$.

By functionally-differentiating $\Xi$ with respect to $z(r)$, and utilizing the mathematical theorem concerning the functional derivative,

${\delta z(r) \over {\delta z(r')}}=\delta (r-r')$,

we get the following equations with respect to the density pair correlation functions.

$\rho (r)={\delta \ln \Xi \over {\delta \ln z(r)}}$,

$\rho ^{(2)}(r,r')={\delta ^{2}\ln \Xi \over {\delta \ln z(r)\delta \ln z(r')}}$.

A relation between $\rho (r)$ and $\rho ^{(2)}(r,r')$ can be obtained after some manipulation as,

${\delta \rho (r) \over {\delta \ln z(r')}}=\rho ^{(2)}(r,r')-\rho (r)\rho (r')+\delta (r-r')\rho (r).$

Now, we define the direct correlation function by an inverse relation of the previous equation,

${\delta \ln z(r) \over {\delta \rho (r')}}={\delta (r-r') \over {\rho (r')}}-c(r,r').$

Inserting these two reults into the chain-rule theorem of functional derivatives,

$\int {\delta \rho (r) \over {\delta \ln z(r^{\prime \prime })}}{\delta \ln z(r^{\prime \prime }) \over {\delta \rho (r')}}dr^{\prime \prime }=\delta (r-r')$,

one obtains the Ornstein-Zernike relation. Thus the Ornstein-Zernike relation is, in a sense, a differential form of the partition function.