Exact eigenvalue equation for a finite and infinite collection of muffin-tin potentials

The integral eigenvalue equation of the Hamiltonian with a finite-range potential is transformed so as to explicitly take into account the particular structure of a potential consisting of a finite collection of nonoverlapping, muffin-tin type individual potentials (scatterers). The separation between structure and potential, thought to be obtained as an exact result in the framework of multiple-scattering theory, is found to represent an approximation which originates in having considered what is only a necessary condition to be both necessary and sufficient. As an application, the equation for the energy levels of a muffin-tin periodic potential is discussed and shown to be represented by the Korringa-Kohn-Rostoker equation only as an approximate result.