U sing both INDO/SCI and INDO/SDCI methods, the UV-visibie spectra of C60 have been calculated. The results are in good agreement with experiment, On the basis of cnorrect electronic spectra, calculations of Ihc nonlinear third-order optical suscepti-bility (yijkl) of C60 are reported. The calculations are performed using the INDO/CI melhod combined with a sum-ver-tates expression for yijkl. The calculated resulls, <y>(-ω, ω, -ω, ω)=730×10-34 esu (ω=1.064μm) and <y>(-2ω; ω, ω, 0)=6.90×10 -34 esu (ω=1.91μm), are in good agreement with some of the reeent obserations too.

We present a theoretical study on the two-photo absorption(TPA) prperties of C60. On the basis of the equilibrium geometry optimized by B3LYP/6-31G method, we employ the ZINDO method combined SOS formula to investigate the second hyperpolarizability and TPA cross section of C60. The calculated result of the real part of the second hyperpolarizability of C6 is in good agreement with the previous calculation and the experimental observation. In the 400-1000nm range of TPA wavelength, we calculated TPA cross sections corresponding to all two photon allowed states. As a result, we find that there is only a TPA cross section maximum-995.7