## FFTXlib, a rewrite and optimisation of earlier versions of FFT related routines inside QE pre-v6

FFTXlib is mainly a rewrite and optimisation of earlier versions of FFT related routines inside Quantum ESPRESSO (QE) pre-v6; and finally their replacement. Despite many similarities, current version of FFTXlib dramatically changes the FFT strategy in the parallel execution, from 1D+2D FFT performed in QE pre v6 to a 1D+1D+1D one; to allow for greater flexibility in parallelisation.

**Practical application and exploitation of the code**

FFTXlib module is a collection of driver routines that allows the user to perform complex 3D fast Fourier transform (FFT) in the context of plane wave based electronic structure software. It contains routines to initialize the array structures, to calculate the desired grid shapes. It imposes underlying size assumptions and provides correspondence maps for indices between the two transform domains.

Once this data structure is constructed, forward or inverse in-place FFT can be performed. For this purpose FFTXlib can either use a local copy of an earlier version of FFTW (a commonly used open source FFT library), or it can also serve as a wrapper to external FFT libraries via conditional compilation using pre-processor directives. It supports both MPI and OpenMP parallelisation technologies.

FFTXlib is currently employed within Quantum Espresso package, a widely used suite of codes for electronic structure calculations and materials modeling in the nanoscale, based on planewave and pseudopotentials.

FFTXlib is also interfaced with “miniPWPP” module that solves the Kohn Sham equations in the basis of planewaves and soon to be released as a part of E-CAM Electronic Structure Library.