Abstract
Zero-point motion of the atoms of a molecule or solid causes renormalization of physical properties. This renormalization depends ort the vibrational amplitudes and thus on the isotopic mass, giving rise to isotope shifts of quantities such as bandgaps and lattice constants. This effect is normally omitted in band-structure calculations. Therefore it is appropriate to compare theory not with experiment but rather with values which are corrected for zero-point renormalization. A simple scheme is presented for doing this by linear extrapolation of high-T thermal shift data back to T = 0.