Abstract
Formulae are derived for the electronic Green function of a system, and in particular of a crystal, when this is expressed as a linear combination of basis functions which are not mutually orthogonal. Expressions are derived for the electronic density of states and the electrical conductivity of a system in this representation, and the differences in these properties between perfect and imperfect crystals, and between different types of crystal, are examined. Application of the method to a simple cubic crystal with the Mathieu potential shows that, as for a one dimensional system, the use of optimized in place of fixed atomic orbitals as basis functions leads to a great increase in accuracy.