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−  For particles acting through twobody central forces alone one may use the [[Thermodynamic relations  thermodynamic relation]]
 +  #REDIRECT[[Pressure#Pressure equation]] 
−   
−  :<math>p = \left. \frac{\partial A}{\partial V}\right\vert_T </math>
 
−   
−  Using this relation, along with the [[Helmholtz energy function]] and the [[partition function  canonical partition function]], one
 
−  arrives at the socalled
 
−  '''pressure equation''' (also known as the '''virial equation'''):
 
−  :<math>p^*=\frac{\beta p}{\rho}= \frac{pV}{Nk_BT} = 1  \beta \frac{2}{3} \pi \rho \int_0^{\infty} \left( \frac{{\rm d}\Phi(r)} {{\rm d}r}~r \right)~{\rm g}(r)r^2~{\rm d}r</math>
 
−   
−  where <math>\beta = 1/k_BT</math>,
 
−  <math>\Phi(r)</math> is a ''central'' [[Intermolecular pair potential  potential]] and <math>{\rm g}(r)</math> is the [[pair distribution function]].
 
−  ==References==
 
−  [[category: statistical mechanics]]
 