Ab initio study of the nonlinear optical properties of urea: Electron correlation and dispersion effects

We 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. © 1995 John Wiley & Sons, Inc.