We present a study of the ionization efficiency in pure crystals based on an extension of Lindhard’s theory, in which the energy given to atomic motion by nuclear recoils is calculated taking into account the atomic binding energy. We construct a modified integral equation that incorporates this effect consistently and find a numerical solution to this equation that leads to a “quenching factor” (QF) which is in good agreement with the available experimental measurements for Si and Ge. We argue that the model with constant binding energy is a good approximation for Ge even for energies close to the true cutoff, while for Si is valid up to recoil energies greater than 400 eV. We also describe recent studies aimed at further extending the calculation of the QF for Si to sub-keV energies, relevant for current and future direct dark matter searches and the detection of coherent elastic scattering of neutrinos off nuclei.