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The influence of a single phosphorous impurity on structural and electronic properties of spherical, diamond-like, hydrogen-passivated, ultrasmall Si[Formula: see text]H[Formula: see text], Si[Formula: see text]H[Formula: see text], Si[Formula: see text]H[Formula: see text] and Si[Formula: see text]H[Formula: see text] nanocrystals with silicon core diameters of 0.88, 1.03, 1.26 and 1.58 nm was studied by density function theory calculations. In this ultrasmall length scale, the dependence of structural deformation and electronic properties with gradually increasing sizes has not been practically investigated. A detailed analysis of the structural deformation and charge distribution initiated by the presence of the impurity is conducted to understand how structural change occurs within this length scale, where quantum confinement effects become predominant. The Si[Formula: see text]P[Formula: see text]H[Formula: see text] nanocrystal with P[Formula: see text] located in its center is completely deformed. In a larger nanocrystal, the spherical surrounding impurity remains and the P–Si bond lengths increase. In Si[Formula: see text]P[Formula: see text]H[Formula: see text], the first sphere is expanded, energies of phosphorous formation in central and subsurface positions differ insignificantly, and the P atom can be located in both the central and subsurface positions. In all nanoparticles, the charges of central P[Formula: see text] atoms are negative. The width of the bandgap in undoped nanocrystals is much larger than in bulk silicon and depends on their sizes. The phosphorous introduces the splitting level in the bandgap located closer to the conduction band.

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