Electron correlation effects in hyperpolarizabilities of p-nitroaniline

The importance of including electron correlation effects in the determination of the equilibrium structure and hyperpolarizabilities of p-nitroaniline is investigated. Second-order perturbation theory gives static hyperpolarizabilities which are 88% to 100% larger than those obtained without electron correlation. These values are scaled by a self-consistent field frequency-dependent contribution for comparison with experimental measurements. Despite the large increase in β(-2ω;ω,ω) from electron correlation, the theoretical result is still 3 times smaller than the experimental value obtained in solution. This difference is discussed. The finite field perturbation approach for calculating hyperpolarizabilities is discussed, and we outline a generalization of a previous method based on the solution of simultaneous equations.