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Van-der-Waals supercritical fluid: Exact formulas for special lines

In the framework of the van-der-Waals model, analytical expressions for the locus of extrema (ridges) for heat capacity, thermal expansion coefficient, compressibility, density fluctuation, and sound velocity in the supercritical region have been obtained. It was found that the ridges for different thermodynamic values virtually merge into single Widom line only at $T<1.07 T_c, P<1.25P_c$ and become smeared at $T<2T_c, P<5P_c$, where $T_c$ and $P_c$ are the critical temperature and pressure. The behavior of the Batschinski lines and the pseudo-Gruneisen parameter $γ$ of a van-der-Waals fluid were analyzed. In the critical point, the van-der-Waals fluid has $γ=8/3$, corresponding to a soft sphere particle system with exponent $n=14$.