The independent gradient model based on Hirshfeld partition (IGMH) is a popular method in graphically displaying occurrence region and nature of non-covalent interactions. It can be seen form Figure that the main interaction region in M + @C18 in becomes wider as atomic radius increases in the order of Li + -Na + -K + . In sharp contrast, K + , Rb formed between M + and C18, as covalent interaction always causes notable electron concentration. This observation is also consistent with the Wiberg bond order analysis, and somewhat explains why K + , Rb + , and Cs + much more tend to be adsorbed inside C18 than Li + and Na + , because the former three will interact with significantly fewer C18 atoms if they are adsorbed outside.