  We discuss the origin and relevance for computer simulations of a strong
finite-size effect that appears when using the Ewald summation formula. It can
be understood as arising from a volume-dependent shift of the potential in a
finite, periodic box relative to the infinite volume limit. This shift is due
to the fact that the ``zero of energy'' for a periodic system cannot be defined
by letting the interacting particles be separated by an infinite distance; the
correct definition corresponds to setting its $\bbox k=\bbox 0$ Fourier mode to
zero. The implications of this effect for computer simulations are discussed.
