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The idea of modelling polyatomic molecules was pioneered by Becke~\cite{Becke_1989}, who used a combination of radial-grids centered around each atom.  In 1994 Chelikowsky, Troullierm and Saad~\cite{Chelikowsky_1994} presented a practical approach for the solution of the Kohn-Sham equations using uniform grids combined with pseudo-potentials. What made the approach competitive was the use of high-order finite differences, that keep the approximation error of the Laplacian on the grid low without requiring very dense meshes.  One of the main advantages of working in real-space is that not pose any requirements to the fields that are to be discretized.  In fact, most basis set representations, including plane-waves and atomic orbitals, must relay on an auxiliary real-space discretization. This is used, for example, for the calculation of the exchange and correlation (XC) term of DFT.