Critical-density compressibility of a cubic gas

For the cubic-density model, ${\kappa }_T=(n\partial p/\partial n{)}^{-1}$ gives ${\kappa }_T(n_c,T)=1/[n_c{k}_B(T-T_c)]$. Divergence from above is a mean-field critical prediction. The negative value below $T_c$ signals an unstable homogeneous branch rather than an equilibrium negative compressibility.