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We present a theoretical study of lattice deformation and its influence on the electronic states and the optical transitions in Si and Ge nanocrystals covered by hydrogen. The lattice of Si and Ge nanocrystals has been optimized by Merck molecular force field method. The semiempirical tight-binding computer simulation has shown that the lattice optimization reduces effec-tive energy band gap in silicon nanocrystals and increases it in germanium nanocrystals. This is similar to the effect of the hydrostatic compression of bulk Si and Ge. The sketch of the state density distribution in k-space is presented. It demonstrates the orbital-valley mixing, that appears due to space-quantization and lattice deformation influence.

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