Electron correlation effects on the nonlinear optical properties of conjugated polyenes

We employ the coupled cluster method to calculate the nonlinear optical coefficients for conjugated polyenes and their push–pull forms. We find that the electron correlation always reduces the nonlinear optical responses. While MP2 can predict correct trend for properties like dipole moment and polarizability, it gives completely wrong trend for the hyperpolarizabilites for long polyenes. It is highly cautioned to use MP2 when benchmarking computational methods for nonlinear properties such as improving exchange-correlation functionals in DFT.