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rnWe present the results of ab initio calculations on the first-, second-, and third-order molecular polarizabilities of urea. An efficacious general finite field perturbation approach, previously applied in the case of paranitroaniline, is extended to the evaluation of axial and nonaxial components of the nonlinear responses. The validity of our numerical procedure is examined at the Hartree-Fock level of theory by comparison with analytical derivative results. The impact of electron correlation is analyzed, by calculating the optical nonlinearities at the Moller-Plesset perturbation theory level. The second-order Moller-Plesset electron correlation correction is shown: (i) to enhance the third-order polarizability y by almost a factor of 2 and (ii) to include the major correlation effects as consideration of the fourth-order correction further improves the y components by less than 20%. We also discuss the frequency-dependence of the nonlinear optical properties of urea by presenting calculations of the dynamic components at the noncorrelated level of theory for different optical processes.

The static first-and third-order polarizabilities of several benzene and thiophene derivatives are evaluated at the ab initio level via an efficacious general finite field approach. The impact of electron correlation is explored by calculating the molecular polarizabilities at the Møller-Plesset second-order perturbation (MP2) theory level using an extended basis set. Further, we examine the influence of molecular architecture on the nonlinear optical response, in particular the switch from an arylethenyl type of structure to a quinoid structure. Finally, we apply the correction vector method combined with the intermediate neglect of differential overlapsingle and double excitation configuration interaction (INDO-SDCI) technique to evaluate the optical nonlinearities at the semiempirical level. The reliability of this procedure is established by comparison with the corresponding high-level ab initio polarizability values. The results clearly show the sensitive dependence of the nonlinear optical properties on electron correlation effects that are found to quantitatively and qualitatively affect the third-order polarizability values.