Case of Less Than Two Degrees of Freedom, Negative Pressure, and the Fermi–Dirac Distribution for a Hard Liquid

The notion of ideal liquid for the number of degrees of freedom less than $2$, i.e., $\gamma<0$, is introduced. The values of the pressure $P$ and of the compressibility factor $Z$ on the spinodal in the negative pressure region for the van der Waals equation determine the value of $\gamma$, $\gamma(T)<0$, for $\mu=0$. For $T\leq \frac{3^3}{2^5} T_c$, a relationship with the van der Waals equation is established.