To explain the ill-behavior of the D3 model for the H2-NiKur case, let's closely examine Figure , where the C6 coefficient for Ni and N atoms is plotted with CN N assumed to be equal to 3 (to mimic the situation in the NiKur cluster). As shown, a sharp change in C6 NiN occurs when transitioning from CN Ni = 3 to CN Ni = 5. In particular, the sharp change occurs at CN Ni ≈ 4. When we compare the CN Ni versus the Ni-H2 distance, we observe that this specific coordination number of Ni is reached at a Ni-H2 distance of 1.632 Å. Thus, as the H2 molecule approaches, the C6 NiN coefficient sharply decreases, leading to a sharp increase in the dispersion energy (keeping in mind the negative sign in the E (2) expression) and the formation of the unphysical minimum.