The frequency - dependent third - order nonlinear optical polarizabilities with different optica processes have been calculatedfor fused benzeniod polycyclic hydrocarbons having from two to seven rrings, in tems of the INDO-SDCI quantum chemical method combining with the sum - over - states method. The calculated dispersions of the third- harmonic generation (THG), electric-field- induced second- harmonic generation (EFISHG) and degeneration four- wave mixing (DFWM) have been discussed, and influences of the π - electron delocalizations upon the third -order NLO responses have been descibed for these hydrocarbons. additionally, the explicit expression of the calculating DFWM polarizability has been presented in this work.It is found from the obtained results that the third- order NLO polarizability gets larger as the numbers of benzene rings increase in a linear arrangement, and π -π ^* electronic charge transfers make significant contributions to the third-order polarizability in these hydrocarbons.

Key words: THIRD DERIVATIVE, FREQUENCY, POLARIZABILITIES, CONFIGURATION, NON LINEAR OPTICS, FUSED RING COMPOUNDS, HYDROCARBON WITH PHENYLS, DISPERSION