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Chemistry LibreTexts

General correlation functions

A general correlation function can be defined in terms of the probability distribution function \(p^{(n)} (r_1, \cdots , r_n)\) according to

\[ g^{(n)}(r_1, \cdots , r_n) = {1 \over p^n} p^{(n)} (r_1, \cdots , r_n)\]

\[ = {V^n N! \over Z_N N^n (N - n)!} \int dr_{n+1} \cdots dr_N e^{-\beta U(r_1, \cdots , r_N)} \]

Another useful way to write the correlation function is

\[ g^{(n)} (r_1, \cdots , r_n) = {V^n N! \over Z_N N^n (N - n)!} \int dr'_1 \cdots dr'_N e^{-\beta U(r_1, \cdots , r_N)} \delta (r_1 - r'_1) \cdots \delta (r_n - r'_n) \]

\[ {V^n N! \over Z_N N^n (N - n)!} \left < \Pi _{i=1}^n \delta (r_i - r'_i \right >_{r'_1, \cdots , r'_N} \]

i.e., the general n-particle correlation function can be expressed as an ensemble average of the product of \(\delta \) -functions, with the integration being taken over the variables \(r'_1, \cdots , r'_N \).