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Visualization of the defect wavefunction for a hydrogen atom interacting with a compound semiconductor. Hydrogen (yellow) breaks a bond between cadmium (green) and tellurium (blue) and binds to the cadmium atom. Transparent spheres indicate atomic positions in the perfect lattice. The pink isosurface shows the defect wavefunction calculated from first principles. Such calculations have enabled establishing a universal alignment for the electronic level of hydrogen (photo and text: C G Van de Walle and J Neugbauer 2003)