ABSTRACT
The cohesive, electronic, optical, magnetic and superconducting properties exhibited by solids are determined principally by the behaviour of electrons in the field of the nuclei of the constituent atoms. This is essentially a many-body problem which is hopelessly difficult to solve without introducing simplifying but reasonable approximations. Different classes of solid-state properties can be explained on the basis of the different levels of sophistication with which one can treat the electron-electron interactions in solids (see, e.g., Ashcroft and Mermin 1976). For instance,
• The free-electron approximation, neglecting discreteness of the ions alto gether.
