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\section{Forces and geometry optimization on real-space grids}  \label{sec:forces}  %Xavier, Alejandro  When a uniform grid is used, the positions of the points break the translation invariance that one would expect from a physical system. This is reflected in that the representation of the potential of an atom sitting on top of a grid point might be slightly different from the potential of the same atom located between points. This implies that a rigid displacement of the system produces an artificial variation of the energy and other properties. If we plot the energy of the atom as a function of this rigid displacement, the energy shows an oscillation that gives this phenomena the name of egg-box effect.