A reduced Green function for finite solids

A reduced Green function, which is essentially the projection of the usual electronic Green function onto a finite-dimensional vector space, is defined for electrons in a finite system. This reduced Green function can be used in place of the usual Green function in formulae for the density of states and conductivity of the system, and can be calculated by any of the techniques used to calculate the usual Green function. Applications of the method to the theory of disordered and amorphous solids are discussed.