Solution of Poisson's equation for finite systems using plane wave   methods
  Reciprocal space methods for solving Poisson's equation for finite charge distributions are investigated. Improvements to previous proposals are presented, and their performance is compared in the context of a real-space density functional theory code. Two basic methodologies are followed: calculation of correction terms, and imposition of a cut-off to the Coulomb potential. We conclude that these methods can be safely applied to finite or aperiodic systems with a reasonable control of speed and accuracy. 
Alberto Castro, Angel Rubio and M. J. Stott