About the Coulomb potential in crystals

The Coulomb potential in a crystal is discussed. It is shown that its Fourier series expansion has a singularity for the V(0, 0, 0) component, which is important when comparing different compounds, or when using the Coulomb potential as a probe for reactivity. Methods to calculate this term are discussed. Sum rules for multipolar moments of crystals in terms of structure factors are derived, which are of interest for the comparison of microscopic and macroscopic dielectric properties.