  The EPW (Electron-Phonon coupling using Wannier functions) software is a
Fortran90 code that uses density-functional perturbation theory and maximally
localized Wannier functions for computing electron-phonon couplings and related
properties in solids accurately and efficiently. The EPW v4 program can be used
to compute electron and phonon self-energies, linewidths, electron-phonon
scattering rates, electron-phonon coupling strengths, transport spectral
functions, electronic velocities, resistivity, anisotropic superconducting gaps
and spectral functions within the Migdal-Eliashberg theory. The code now
supports spin-orbit coupling, time-reversal symmetry in non-centrosymmetric
crystals, polar materials, and $\mathbf{k}$ and $\mathbf{q}$-point
parallelization. Considerable effort was dedicated to optimization and
parallelization, achieving almost a ten times speedup with respect to previous
releases. A computer test farm was implemented to ensure stability and
portability of the code on the most popular compilers and architectures. Since
April 2016, version 4 of the EPW code is fully integrated in and distributed
with the Quantum ESPRESSO package, and can be downloaded through QE-forge at
http://qe-forge.org/gf/project/q-e.
