  Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is
developed. It is demonstrated, that the valley splittings of confined electrons
and holes strongly and peculiarly depend on the geometry of a nanocrystal. When
the nanocrystal lacks a microscopic center of inversion and has T_d symmetry,
the splitting is strongly suppressed as compared to the more symmetric
nanocrystals with O_h symmetry, having an inversion center.
