cell fluid model equation of state - collective variables Morse potential coexistence curve Morse interaction potential Widom line first-order phase transition grand partition function комiркова модель потенцiал Морзе потенцiал взаємодiї Морзе Phase behavior of a simple cell fluid model asymmetric binary mixture cell model critical point critical region critical temperature density fluctuations equation of state with allowance for Gaussian fluctuations isothermal compressibility iзотермiчна стисливiсть recurrence relations reference system simple fluid supercritical region thermal expansion thermodynamic potential thermodynamic response functions