  We study the behaviour of total-energy supercell calculations for dipolar
molecules and charged clusters. Using a cutoff Coulomb interaction within the
framework of a plane-wave basis set formalism, with all other aspects of the
method (pseudopotentials, basis set, exchange-correlation functional)
unchanged, we are able to assess directly the interaction effects present in
the supercell technique. We find that the supercell method gives structures and
energies in almost total agreement with the results of calculations for finite
systems, even for molecules with large dipole moments. We also show that the
performance of finite-grid calculations can be improved by allowing a degree of
aliasing in the Hartree energy, and by using a reciprocal space definition of
the cutoff Coulomb interaction.
