where a and b can refer to molecules belonging to the same or distinct species. R A represents the positions of all atoms of all molecules belonging to species A, R a represents the positions of all atoms in one particular molecule a belonging to species A, r ij = |r i -r j | is the atom-atom distance between atoms i and j with positions r i and r j , respectively, q i is the fixed point charge of atom i, and ϵ ij and σ ij are LJ parameters corresponding to the atom types of atoms i and j.