We propose, using density functional, configuration interaction, and quantum Monte Carlo calculations, structural prototypes of ultrasmall ultrabright particles prepared by dispersion from bulk. We constructed near spherical structures (Td point group symmetry) that contain 29 Si atoms, five of which constitute a tetrahedral core and the remaining 24 constitute a hydrogen terminated reconstructed Si surface. The surface is a highly wrinkled or puckered system of hexagons and pentagons (as in a filled fullerene). We calculated, for several surface reconstruction models, the coordinates of atoms, the absorption spectrum, the absorption edge, polarizability, and the electron diffraction pattern. The Si29H24 (six reconstructed surface dimers) gives a size of 0.9 nm, an absorption spectrum and bandgap (3.5±0.3 eV), in fair agreement with measurement. The structure yields a polarizability of 830 a.u. with an effective “dielectric” constant of ∼6.0. The calculated electron diffraction of single particles shows residual crystalline coherent scattering for large but not small scattering angles. © 2001 American Institute of Physics.