Optimal Fourier filtering of a function that is strictly confined within   a sphere
  We present an alternative method to filter a distribution, that is strictly confined within a sphere of given radius $r_c$, so that its Fourier transform is optimally confined within another sphere of radius $k_c$.   In electronic structure methods, it can be used to generate optimized pseudopotentials, pseudocore charge distributions, and pseudo atomic orbital basis sets. 
Jose M. Soler and Eduardo Anglada