Abstract
A formal treatment has been given of the surface states associated with a semi-infinite crystal. The treatment assumes that the surface states are produced by a potential which is a function only of the distance from the surface but is otherwise unspecified. A formal expansion is made in a complete set of Wannier orbitals, but the conclusions of the paper are based on the assumptions that (a) it is sufficient to use the orbitals of a single band, and (b) the tight-binding approximation is adequate for that band. Some theorems are given on the localization of Wannier orbitals, the localization of surface states, and the number of surface states for a given penetration of the surface potential.

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