Figure 1.

Split of U_LJ into U_st and U_na at r_ij = d_ij, when only a pair of atoms is considered. If the two atoms are of the same type, then d_ij = σ_ij; the latter is the interatomic distance where U_LJ = 0. For two different types of atoms, d_ij is slightly larger than σ_ij, and hence when the split is triggered, U_na would be slightly negative instead of 0.