Madelung summations: especially in bulk systems

When routines such as ATOMIC and VMULTI take a long time, visible in the log file, you can try to reduce the Madelung screening parameter. Band uses fit functions to calculate the Coulomb potential of the neutral and small deformation density. You can see how large Band has chosen this parameter in the output

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1                            **  Bloch Summations  **



 Cut off criterion                                        0.100E-03
 Max. extension of the functions                         17.9095
 Max. cell distance taken                                37.7560
 Nr. of cells                                            19

 Fermi parameters for the Coulomb potential:
   Break off point                                       20.0000
   Fall off parameter                                     2.2361

This is example output for a slab. Here the Coulomb interaction screening parameter has been set to 20 bohr, leading to a reasonable number of cells. For large 3d cells you can get an incredible amount of cells, due to an unnecessary large choice of the screening parameter. See key Screening%RMadel.