Calculation of electron-phonon interaction coefficients

The calculation of electron-phonon coefficients in metals is made difficult by the slow convergence of the sum at the Fermi energy. It is convenient to calculate phonons, for each q-vector of a suitable grid, using a smaller k-point grid, saving the dynamical matrix and the self-consistent first-order variation of the potential (variable fildvscf). Then a non-SCF calculation with a larger k-point grid is performed. Finally the electron-phonon calculation is performed by specifying elph=.true., trans=.false., and the input files fildvscf, fildyn. The electron-phonon coefficients are calculated using several values of gaussian broadening (see PH/elphon.f90) because this quickly shows whether results are converged or not with respect to the k-point grid and Gaussian broadening. See Example 07.

All of the above must be repeated for all desired q-vectors and the final result is summed over all q-vectors, using pwtools/lambda.x. The input data for the latter is described in the header of pwtools/lambda.f90.