c_g = - \frac{d}{dp} \left( \ln \left( \frac{Z}{p} + \ln (\nu R T) \right) \right) = - \frac{p}{Z} \frac{d}{dp} \left( \frac{Z}{p} \right) =
- \frac{p}{Z} \left( \frac{1}{p} \frac{dZ}{dp} - \frac{Z}{p^2} \right) = \frac{1}{p} - \frac{1}{Z} \frac{dZ}{dp} |