Abstract
The theory of the one-electron energy spectrum in the presence of weak disorder is discussed using Green's-function language. The emphasis is on features which enter for Bloch band electrons but are absent for plane-wave free electrons. Static impurity disorder and dynamic phonon disorder are given parallel formulations. At high temperatures, phonon disorder can be treated statically (the adiabatic approximation) Migdal's theorem does not apply in this regime. A new scheme is proposed for including electron renormalization due to high-temperature phonon disorder in a pseudopotential band calculation. In the following paper, optical properties are discussed.