Keywords: user_guide

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## 4.3 Calculation of electron-phonon interaction coefficients

Since v.5.0, there are two ways of calculating electron-phonon coefficients, distinguished according to the value of variable `electron_phonon`. The following holds for the case `electron_phonon=` `'interpolated'` (see also Example 03).

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 use a coarse k-point grid to calculate phonons on a suitable wavevector grid; a dense k-point grid to calculate the sum at the Fermi energy. The calculation proceeds in this way:

1. a scf calculation for the dense -point grid (or a scf calculation followed by a non-scf one on the dense -point grid); specify option `la2f=.true.` to `pw.x` in order to save a file with the eigenvalues on the dense k-point grid. The latter MUST contain all and + grid points used in the subsequent electron-phonon calculation. All grids MUST be unshifted, i.e. include = 0.
2. a normal scf + phonon dispersion calculation on the coarse k-point grid, specifying option `electron_phonon='interpolated'`, and the file name where the self-consistent first-order variation of the potential is to be stored: variable `fildvscf`). The electron-phonon coefficients are calculated using several values of Gaussian broadening (see `PHonon/PH/elphon.f90`) because this quickly shows whether results are converged or not with respect to the k-point grid and Gaussian broadening.
3. Finally, you can use `matdyn.x` and `lambda.x` (input documentation in the header of `PHonon/PH/lambda.f90`) to get the α2F(ω) function, the electron-phonon coefficient λ, and an estimate of the critical temperature Tc.

See the appendix for the relevant formulae. Important notice: the q→ 0 limit of the contribution to the electron-phonon coefficient diverges for optical modes! please be very careful, consult the relevant literature.

Next: 4.4 DFPT with the Up: 4 Using PHonon Previous: 4.2 Calculation of interatomic Contents