**4. Conclusions**

This study has shown that the analysis of the critical points of the Laplacian of the charge density could be informative in revealing the actual nature of the surface reactivity of the chlorine atom in CH3Cl. This is in line with the nature of the local extrema of electrostatic potential identified on the surface of the Cl atom in CH3Cl using the MESP model. In particular, it is shown that the combination of a suitable isodensity envelope with an appropriate theoretical method is important to correctly identify the electrophilic nature the Cl atom in CH3Cl.

The electronic charge density distributions around the lateral and axial sites of Cl in CH3Cl is not isotropic, indicating the amphiphilic nature of the Cl atom. The negative lateral sites on the Cl are shown to display su fficient ability to attract positive sites on the interacting atoms to form halogen bonds, or chalcogen bonds, or hydrogen bonds.

The attractive interaction of the positive "hole" on the Cl atom in CH3Cl with various "lumps" in the interacting bases has led to the conclusion that the positive electrostatic potential on the Cl is certainly not induced by the electric field of the interacting species as others have suggested [2,3,10,28]. Rather, it is an inherent property of this atom in the molecule.

The bond path and critical point topologies of QTAIM associated with the primary bonding interactions in the 18 complexes are shown to be consistent with an RDG isosurface analysis. Although these topologies did not appear between the weakly bound atoms in some complexes, the results of QTAIM's delocalization analysis were shown to be concordant with those of RDG.

The supermolecular and SAPT interaction energies were shown to be in agreement. The dispersion interaction was also shown to be one the most important driving forces responsible for the formation of the 18 complexes investigated.

As shown for the complexes between CH3Cl and SO, all types of intermolecular contacts cannot be regarded as σ-hole interactions.

**Author Contributions:** Conceptualization and Project Design, P.R.V. and A.V.; Investigation, P.R.V. and A.V.; Supervision, P.R.V.; Writing—Original Draft, P.R.V. and A.V.; Writing—Review and Editing, P.R.V., A.V., and H.M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was entirely conducted using the various facilities provided by the University of Tokyo. P.R.V. is currently a ffiliated with AIST and thanks Koichi Yamashita for support. H.M.M. thanks the National Research Foundation, Pretoria, South Africa, and the University of the Witwatersrand for funding.

**Conflicts of Interest:** The authors declare no conflicts of interest.
