Where 𝑵 is the total atom number in the system studied, 𝝆 𝟎 the average number density of atoms, 𝒓 𝒊𝒋 the distance between atom 𝒊 and atom 𝒋, and 𝜹(𝒓 -𝒓 𝒊𝒋 ) the Dirac-delta function, which equals a single unit only when 𝒓 = 𝒓 𝒊𝒋 . The above equation illustrates the meaning of PDF, while each atom-atom distance could not be directly measured in a real experiment. The reduced PDF (rPDF) was commonly used to describe the structure features. This correlation function can be written as: