The behavior of D XY (Q) with an increase of the charge Q depends on the sign of ∆η. To determine it, we need to know if the hardness of the neutral parent ("P") or the reduced hardness of the neutral fragments ("X" and "Y") is larger. From Equation , we know that the hardness is half of the HOMO-LUMO gap of the neutral molecule, which means that the hardness should be a positive number (η > 0). Furthermore, the HOMO-LUMO gap usually tends to decrease with increasing size of the molecular system. This means that the fragments would have equal or greater hardness compared to the parent (η X , η Y ≥ η P ). This inequality leads us to (see Equation )