  It is shown that Coulomb series are to be considered within a special mode of
summation so as to describe bulk properties of crystals. The translational
invariance is then an explicit integral property of Coulomb series that is
tantamount to the effect of invariant periodic boundary conditions discussed
earlier. Absolute bulk potentials with zero mean value are then substantiated
as a unique solution in the general case of triclinic lattices. An invariant
treatment of the bulk Coulomb energy follows therefrom. The potential symmetry
is verified for simple point-charge lattices and is connected with the centre
of gravity of the potential field that is relevant to non-local charges as
well.
