GWW (GW Calculations using Wannier functions), is a recent addition to the Quantum Espresso package. Developed by Dr. Paulo Umari, the code calculates polarisation using a basis of localised Wannier orbitals within the GW approximation, an approach which is around two orders of magnitude faster than convential plane wave basis methods.

The type of calculations carried out by the Sheffield group involve a workflow of 7 individual jobs. Initial investigations showed that these calculations were dominated by the calculation of the dielectric matrix (in particular the head_nscf step of the calculation).

Output from the inbuilt timing routines in the code indicated that for the longest running parts of the calculation (head_nscf), and also for the other parts of the calculation, a large fraction of the calculation is spent in performing a 3D FFT (typically over 50%), and that much of this was `MPI_Alltoallv` communication derived from the FFT transpose.