Replicate or alternate? Isotropic boundary conditions and implications for Ewald summation

Periodic boundary conditions (PBC) are well suited to describe repetitive structures in space, yet flawed for isotropic setups in which it is commonly used. We present an approach based on exploiting the congruency of a unit cell which combines the periodic nature of PBC with isotropy of an alternating cell. By applying these isotropic periodic boundary conditions (IPBC) to the standard Ewald summation for electrostatic interactions, we show a marked reduced artificial ordering and a need for significantly less wave vectors to an otherwise equivalent PBC system. The methodology can be trivially implemented in existing molecular dynamics or Metropolis Monte Carlo Ewald summation codes, and is applicable also for particle mesh Ewald summation.