(color online) Comparison of valence atomic energy levels with the acceptor impurity binding energies in Si. (a) Valence atomic energy levels calculated using a density functional theory within a local density approximation for most group II–VI elements (provided by Wei^[19]). (b) Left: p valence electron energy levels of Group III elements with respect to that of Si 3p energy level (from the graph in panel (a)); right: experimental values of the acceptor impurity binding energies of Group III elements in Si.