Shallow defects in semiconductors are of major importance, both scientifically and technologically. These include donors, acceptors, isoelectronic impurities and various clusters, and may involve trapped carriers or bound excitons. Hopfield provided the key to understanding the more complicated defects by stressing their relation to atomic and molecular systems. The authors have developed a general computer program, SEMELE to exploit this relation further. SEMELE provides a systematic and straightforward way of performing self-consistent calculations on shallow defects and is particularly suited to donor-acceptor pair and related systems. The main assumptions are discussed, together with the various ways of treating central-cell corrections. Results are given for excitons bound to neutral donors in GaP
