  Critical exponent $\gamma \succeq 1.1$ characterizes behavior of the
mechanical susceptibility of a real fluid when temperature approaches the
critical one. It results in zero Gaussian curvature of the local shape of the
critical point on the thermodynamic equation of state surface, which imposes a
new constraint upon the construction of the potential equation of state of the
real fluid from the empirical data. All known empirical equations of state
suffer from a weakness that the Gaussian curvature of the critical point is
negative definite instead of zero.
