During the past few years, there has been dramatic progress in theoretical and computational studies of large molecules and local ized states in solids. Various semi-empirical and first-principles methods well known in quantum chemistry have been applied with considerable success to ever larger and more complex molecules, including some of biological importance, as well as to selected solid state problems involving localized electronic states. In creasingly, solid state physicists are adopting a molecular point of view in attempting to understand the nature of electronic states associated with (a) isolated structural and chemical defects in solids; (b) surfaces and interfaces; and (c) bulk disordered solids, most notably amorphous semiconductors. Moreover, many concepts and methods already widely used in solid state physics are being adapted to molecular problems. These adaptations include pseudopotentials, statistical exchange approxi mations, muffin-tin model potentials, and multiple scattering and cellular methods. In addition, many new approaches are being de vised to deal with progressively more complex molecular and local ized electronic state problems.
