# Introduction to Mathematical Physics/N body problems and statistical equilibrium/Van der Waals gas

Van der Waals\footnote{Johannes Diderik van der Waals received the physics Nobel prize in 1910.} gas model\index{Van der Walls gas} is a model that also relies on classical approximation, that is on the repartition function that can be written:
$Z={\frac {1}{N!}}{\frac {1}{h^{3N}}}\int \int d^{3}p_{1}\dots d^{3}p_{N}d^{3}r_{1}\dots d^{3}r_{N}e^{-{\frac {H}{k_{B}T}}}$
where the function $H(p_{1},\dots ,p_{N},r_{1},\dots ,r_{N})$ is the hamiltonian of the system.
$H(p_{1},\dots ,p_{N},r_{1},\dots ,r_{N})={\frac {1}{2m}}\sum p_{i}^{2}+U(r_{1},\dots ,r_{N})$
As for the perfect gas, integration over the $p_{i}$'s is immediate:
$Z={\frac {1}{N!}}\left({\frac {2\pi mk_{B}T}{h^{2}}}\right)^{\frac {3N}{2}}Y$
with
eqYint
$Y=\int d^{3}q_{1}\dots d^{3}q_{N}e^{-{\frac {U(r_{1},\dots ,r_{N})}{k_{B}T}}}$
We can simplify the evaluation of previous integral in neglecting correlations between particles. A particle number $i$ doesn't feel each particle but rather the average influence of the particles cloud surrounding the considered particle; this approximation is called mean field approximation \index{mean field}. It leads, as if the particles were actually independant to a factorization of the repartition function. Interaction potential $U(r_{1},\dots ,r_{N})$ becomes:
$U(r_{1},\dots ,r_{N})={\frac {1}{2}}\sum U_{e}(r_{i})$
where $U_{e}(r_{i})$ is the "effective potential" that describes the mean interaction between particle $i$ and all the other particles. It depends only on position of particle $i$. Function $Y$ introduced at equation ---eqYint--- can be factorized:
$Y=\left(\int dre^{-{\frac {U_{e}(r)}{2k_{B}T}}}\right)$
In a mean field approximation framework, it is considered that particle distribution is uniform in the volume. Effective potential has thus to traduce an mean attraction. Indeed, it can be shown that at large distances two molecules are attracted, potential varying like ${\frac {1}{r^{6}}}$. This can be proofed using quantum mechanics but this is out of the frame of this book. At small distances however, molecules repel themselves strongly. Effective potential undergone by a particle at position $r=0$ can thus be modelled by function:
${\begin{matrix}U_{e}(r)&=&\infty {\mbox{ if }}r<r_{0}\\U_{e}(r)&=&-C{\mbox{ if }}r>r_{0}\\\end{matrix}}$
Quantity $Y$ is
$Y=\left((V-bN)e^{\frac {aN}{k_{B}TV}}\right)^{N}$
Quantity $bN$ represents the excluded volume of the particle. It is proportional to $N$ because $N$ particles occupying each a
volume $b$ occupy a $Nb$. On another hand mean potential $U_{0}$ felt by the test particle depends on ratio $N/V$. Usually, one sets:
$U_{0}=-2a{\frac {N}{V}}$
Introducing
$F=-k_{B}T\log Z$
and using
$p=-{\frac {\partial F}{\partial V}}$
one finally obtains Van der Waals state equation:
$\left(p+a{\frac {n^{2}}{V^{2}}}\right)(V-bN)=Nk_{B}T$
Van der Waals model allows to describe liquid--vapour phase transition.
When temperature is lower than critical temperature $T_{c}$, the energy of the system for a given volume $V_{M}$ with $V_{A_{0}}<V_{M}<V_{B_{0}}$ is not $F_{v}(V_{M})$. Indeed, system evolves to a state of lower energy (see figure figvanapres) with energy:
$F(V_{M})=F_{A_{0}}+{\frac {V_{M}-V_{A_{0}}}{V_{B_{0}}-V_{A_{0}}}}(F_{B_{0}}-F_{A_{0}})$
The apparition of a local minimum of $F$ corresponds to the apparition of two phases.\index{phase transition}