Atomic configurations of Pt–Pd–Cu–Au quaternary alloy NPs with (a) 443 atoms and (b) 1417 atoms. For each pair of columns, the first and the second columns represent the whole and the cross-section atomic arrangements of the NPs, respectively. Pt atoms are in orange, Pd atoms in red, Cu atoms in gray, and Au atoms in yellow. Panels (c) and (d) show the corresponding pair correlation functions.