This is the library ScaFaCoS ("Scalable Fast Coulomb Solver").

ScaFaCoS is a parallel library for solving the Coulomb problem. It
provides various state-of-the-art methods with a common interface.

Further details on the project, downloads, online documentation 
as well as support contact data are given on the project's web page

  www.scafacos.de

After invoking 'make doc', the User's Guide of the ScaFaCoS 
library is found in the file 

  doc/manual.pdf

The Git repository at https://github.com/scafacos/scafacos.git uses
submodules to integrate the sources of the PFFT and PNFFT libraries.
To clone the entire ScaFaCoS repository including the submodule sources,
use the command

  git clone --recursive https://github.com/scafacos/scafacos.git

or to update the missing submodules sources (i.e., after normal cloning)

  git submodule update --init --recursive