The electrostatic potential of a periodic lattice
  We calculate the electrostatic potential of a periodic lattice of arbitrary extended charges by using the Cartesian multipole formalism. This method allows the separation of the long-range potential from the contact potential (potential on the source). We write both the electrostatic potential and the interaction energy as convergent sums in the reciprocal space. 
G. Vaman
physics.class-ph cond-mat.mtrl-sci