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 n onaxial components of the nonlinear responses. The validity of our num erical procedure is examined at the Hartree-Fock level of theory by co mparison with analytical derivative results. The impact of electron co rrelation is analyzed, by calculating the optical nonlinearities at th e Moller-Plesset perturbation theory level. The second-order Moller-Pl esset electron correlation correction is shown: (i) to enhance the thi rd-order polarizability gamma by almost a factor of 2 and (ii) to incl ude he major correlation effects as consideration of the fourth-order correction further improves the gamma components by less than 20%. We also discuss the frequency-dependence of the nonlinear optical propert ies of urea by presenting calculations of the dynamic components at th e noncorrelated level of theory for different optical processes. (C) 1 995 John Wiley & Sons, Inc.