# Pair Distribution Function

For a system with N particles, the *configurational distribution function*, <math>F_N(r_1,...,r_N)</math>, of the system appears in the nature of a probability density and satisfies the normalizaiton condition

<math>\int F_N(r_1,...,r_N) d^3r_1 ... d^3r_N =1</math>.

Integrating <math>F_N(r_1,...,r_N)</math> over the coordinates <math>r_2</math>, ..., <math>r_N</math> and multiplying the result by *N*, a single particle distribution function can be obtained

<math>F_1(r_1)= N \int F_N(r_1,...,r_N) d^3r_2 ... d^3r_N =n(r_1)</math>.

The function <math>F_1(r_1)</math> represents the particle density at the point <math>r_1</math>. The two-particle distribution function is now defined as

<math>F_2(r_1,r_2)= N (N-1) \int F_N(r_1,...,r_N) d^3r_3 ... d^3r_N =n^2 g(r)</math>,

where <math>r=r_2-r_1</math>. The above equation defines the *pair distribution funciton* <math>g(r)</math> of the system. The product <math>g(r) d^3r</math> determines the probability of finding a particle in the volume element <math>d^3r</math> around the point <math>r</math> when there has been a particle at the point <math>r=0</math>