cell fluid model equation of state - Morse potential collective variables coexistence curve потенцiал Морзе Morse interaction potential Widom line first-order phase transition grand partition function комiркова модель потенцiал взаємодiї Морзе Curie–Weiss-type potential Phase behavior of a simple cell fluid model asymmetric binary mixture attraction strength cell model critical point critical point parameters critical region critical temperature density fluctuations equation of state with allowance for Gaussian fluctuations isothermal compressibility iзотермiчна стисливiсть phase transitions recurrence relations reference system simple fluid