Grand-canonical ensemble, integral equations

In applying the compressibility equation (3.109), care must be exercised to use the pair correlation function g(R) as obtained in the grand canonical ensemble, rather than the corresponding function g(R) obtained in a closed system. Whenever this distinction is important, we use the notation gQ (R) and gc(R) for open and closed systems, respectively. Although the difference between the two is in a term of the order of AT 1 this small difference becomes important when integration over the entire volume is performed as in the definition of the quantity G (equation 3.110). 

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