The importance of diffuse functions in basis sets to produce reliable 3D pictures of dual descriptor

The finite difference approximation (FDA) allows us to compute dual descriptor correctly because it takes into account the relaxation effects that inner molecular orbitals experience. When these effects are irrelevant, we can use the frontier molecular orbital approximation (FMOA). Although FMOA omits relaxation effects and not FDA, both procedures should not yield too much different 3D representations of the dual descriptor. If so, the cause of such result lies in the fact that the respective basis set lacks diffuse functions, so artificially exacerbating the absence of relaxation effects aforementioned and yielding a remarkable inconsistency between FDA and FMOA.