Oops found the mistake.
\left(\frac{dP}{dV}\right)_{T}=-\frac{RT}{\left(b-V\right)^{2}}+\frac{a\left(\epsilon b + \sigma b + 2V\right)}{\left(V+\epsilon b\right)^{2}\left(V+\sigma b\right)^{2}}
I am trying to teach myself some chemical thermodynamics, in particular some methods for fluid property evaluation. I am reading an excellent book which has helped a lot ("Introduction to Chemical Engineering Thermodynamics", Smith-Van Ness-Abbott, Ed. 7, ISBN: 007-124708-4).
One thing that...