**5. Conclusions**

In the absence of any replacements of H atoms, the chalcogen bond is the strongest followed in order by halogen, pnicogen, and tetrel. Methyl substitution on the Lewis acid weakens all bonds, particularly the halogen bond, which is the weakest of the four in this bonding environment. All bonds are strengthened by fluorosubstitution (peripheral to, rather than opposite the base), which leads to the bonding order: tetrel > pnicogen > chalcogen. The most dramatic bond enhancement arises from replacement of the atom opposite the base by F, and yet a different order of halogen > chalcogen > pnicogen > tetrel. If the third-row Lewis acid atoms are replaced by their second-row analogues, there is a general weakening of the noncovalent bonds, but this change is not consistent from one sort of bond to the next.

**Supplementary Materials:** The following are available online. Optimized coordinates of complexes and monomers. **Author Contributions:** S.S. conceived of the idea for this project and wrote a first draft of the manuscript; W.D. carried out the calculations and compiled the data; Q.L. supervised the calculations and helped with a final draft.

**Funding:** This research was funded by the National Natural Science Foundation of China grant number 21573188.

**Acknowledgments:** APC was sponsored by MDPI.

**Conflicts of Interest:** The author declares no conflict of interest.

## **References**


**Sample Availability: Sample Availability**: Not available.

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