For Noble Gases, Energy is Positive for the Gas Phase, Negative for the   Liquid Phase
  We found from experimental data that for noble gases and H$_2$, the energy is positive for the gas phase, and negative for the liquid, possibly except the small vicinity of the critical point, about $(1- T/T_c) \le 0.005$. The line $E=E_c$, in the supercritical region is found to lie close to the Widom line, where $E_c$ is the critical energy. 
Nobu-Hiko Asanuma
cond-mat.other cond-mat.stat-mech physics.class-ph