dUsing quantum chemical molecular dynamics geometry optimization calculations of the second-order optical susceptibilities for C,,,Si and C, Si, molecules were done. We have found that the substitution by two silicon atoms favors larger second-order optical susceptibility comparing with that one for the one-atom substitution. For the C58Si2 molecule, the indicatrice of the photoinduced second-order susceptibility is more isotropic comparing with the one atom substitution. The results obtained for the second-order nonlinear optical susceptibilities using computer simulations for C59Si and C58Si2 molecules are in good agreement with the experimental photoinduced optical second harmonic generation data. (C) 2002 Elsevier Science B.V. All rights reserved.