Correlation functions for estimating effects of the physical cluster formation

Two correlation functions for estimating effects of the physical cluster formation on features of a fluid must satisfy a system of two integral equations which is equivalent to the Ornstein-Zernike equation and the sum of the two correlation functions is equivalent to the pair correlation function. A specific effect of the physical cluster formation persuades the dependence of their sum on the distance r between particular pair particles to develop a deviation from the dependence which is expressed as the product of the reciprocal of r and a particular function given as the Taylor series due to powers of r. The use of the two correlation functions allows the formation of extremely large physical clusters to be predicted at least near the triple point. The two correlation functions can contribute to examining a feature of a fluid in a specific situation where an effect of the physical cluster formation are considerable.