Ornstein-Zernike relation from the grand canonical distribution function

Defining the local activity by $z(r)=z\exp[-\beta \psi (r)]$ 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

Failed to parse (unknown function "\b"): {\displaystyle \Xi=\sum_N^\infty{1\over N!}\int\dots\int \prod_i^Nz({\b f r}_i)\exp(-\beta U_N){\rm d}{\bf r}_1\dots{\rm d}{\bf r}_N.}

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

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

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

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

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

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

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

Now, we define the direct correlation function by an inverse relation of Eq. (\ref{deltarho}),

Failed to parse (unknown function "\label"): {\displaystyle {\delta \ln z({\bf r})\over{\delta\rho({\bf r'})}}={\delta({\bf r}-{\bf r'})\over{\rho({\bf r'})}} \label{deltalnz}-c({\bf r,r'}).}

Inserting Eqs. (\ref{deltarho}) and (\ref{deltalnz}) into the chain-rule theorem of functional derivatives,

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

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