A powerful computational scheme is presented for calculating the static properties of light interstitials in metallic hosts. The method entails (i) the construction of the potential-energy field using the quasiatom concept, (ii) the wave-mechanical solution of the impurity distribution ("zero-point motion"), (iii) calculation of the forces exerted on the adjacent host atoms and their displacements, and (iv) iteration to self-consistency. We investigate self-trapping phenomena in bcc and fcc metals in detail, and calculate both the ground and low-lying excited states. Implications of the wave-mechanical or band picture to diffusion mechanisms and inelastic scattering experiments are discussed. Impurities treated are +, H, D, T, and He, and particular attention is paid to isotope effects among the hydrogenic impurities. It is argued that especially for + and H the quantum nature of the impurity is crucial. The calculated results are in agreement with a wealth of experimental data.