The second order nonlinear-optical coefficients of a series of substituted polyenes have been calculated by the semi-empirical CNDO/S-CI method. The results show that the introduction of the electronic donor or acceptor and the increase in the distance between the substituents and the length of the conjugated chain may enhance nonlinear optical response. It is also shown that the calculated In bvec is linear with the no. of double bonds in planar and linear trans-polyenes. A saturation effect in substitution is discovered. The theor. explanations are presented for these results. Based on the above discussion one can get the guidelines for synthesis of mols. with larger SHG (second harmonic generation) coefficients

Key words: CNDO APPROXIMATION, SUBSTITUENT EFFECT