Strictly planar structure makes the components of the vec(λ) of M + @C18 in (M + = Li + , Na + , K + , and Rb + ) in the z direction zero, while in contrast, the small vec(λ) of Cs + @C18 in is mainly contributed by its z-component, as Cs + is located vertically outside the ring. It can be seen that different alkali-metal ions, due to their different radii and properties, can composite with C18 to regulate the first hyperpolarizability of the system.