Matjes has been interfaced with Fortran 2003 libraries and uses the C_bindings with different external libraries such as FFTW3 and the Sparse matrix solvers.

for explanations see: For the explanations see the web page of the FFTW3 libraries and especially the page that describe exactly with FFTW compute:
FFTW3 howto

The FFTW libraries are very useful to calculate convolution products of the type

with the product of the Fourrier Transform . For Fourier transform of the convolution products of the type can be written

This type of product appears all the time in the calculation of the correlations or the dipole dipole interaction.