As for the case of vec(λ), the M + @C18 in with different metal ions exhibits different behavior, wherein Li + @C18 in has the highest value, Na + @C18 in has a smaller value, K + @C18 in and Rb + @C18 in have a value almost zero, and Cs + @C18 in has a very small value. Obviously, the vec(λ) of these complexes is closely related to their molecular symmetry, that is, the larger the molecular deviation from the centrosymmetric structure, the greater the vec(λ) value. The component of vec(λ) in the long axis of M + @C18 in with M + = Li + and Na + dominates the contribution to the total first hyperpolarizability, while the components in the molecular plane of M + @C18 in with M + = K + and Rb + is essentially zero due to the approximate centrosymmetric structure.