The most common definition of a correlation function is the canonical ensemble (thermal) average of the scalar product of two random variables, s 1 and s 2 , at positions R and R + r and times t and t + τ τ : C ( r , τ τ ) = ⟨ ⟨ s 1 ( R , t ) ⋅ ⋅ s 2 ( R + r , t + τ τ ) ⟩ ⟩ − − ⟨ ⟨ s 1 ( R , t ) ⟩ ⟩ ⟨ ⟨ s 2 ( R + r , t + τ τ ) ⟩ ⟩ .