A substituted diphenyldisilane having a dimethylamino donor and a perfluoroalkylsulfonyl acceptor shows a moderate second-order nonlinear optical efficiency while being transparent in the visible spectrum. Semiempirical quantum chemical approaches (AM1, ZINDO) as well as an ab initio sum-over-orbitals method (HYPERPOL) were employed to calculate the properties of this compound, of its fragments, and of model molecules, such as monosilyl- and disilanylphenyls substituted with either a donor or an acceptor group and donor-acceptor-substituted benzene. The calculated values of the first hyperpolarizability, beta, show the proper trends when compared with experimental values obtained by hyper-Rayleigh scattering. A simple two-level approach overestimates beta but predicts the trends correctly. The HYPERPOL method is found to produce too low beta values for compounds containing second-row elements; still it yields a useful prediction of experimental trends. The comparison of excited states obtained from ZINDO calculations with experimental absorption spectra suggests that d-orbitals on sulfur should be included in these calculations; the results are less conclusive with respect to d-orbitals on silicon. Molecular geometry was considered, in particular the orientation of donor and acceptor groups with respect to the phenyl ring, and that of the phenyl rings with respect to the Si-Si bridge. Attention is paid to a possible pi-sigma conjugation in disilanes. ZINDO apparently underestimates such interactions. For the diphenyldisilane donor-acceptor compound, this method predicts a partial charge transfer, from the donor to the disilanylene bridge, associated with the excitation that contributes most to beta.