4.5 Calculation of electron-phonon interaction coefficients with the tetrahedron method

When you perform a calculation of electron-phonon interaction coefficients with the tetrahedron method, you have to use an offset q-point grid in order to avoid a singularity at q = Γ; you can perform this calculation as follows:

1. Run pw.x with occupation = "tetrahedra_opt" and K_POINT automatic.
2. Run ph.x with lshift_q = .true. and electron_phonon = "" (or unset it) to generate the dynamical matrix and the deformation potential (in _ph*/{prefix}_q*/) of each q.
3. Run ph.x with electron_phonon = "lambda_tetra". You should use a denser k grid by setting nk1, nk2, and nk3. Then lambda*.dat are generated; they contain λ_qν.
4. Run alpha2f.x with an input file as follows:

   &INPUTPH
   ! The same as that for the electron-phonon calculation with ph.x
    :
   /
   &INPUTA2F
     nfreq = Number of frequency-points for a2F(omega), 
   /
   

   Then λ, and ω_ln are computed and they are printed to the standard output. α²F(ω) and (partial) phonon-DOS are also computed; they are printed to a file prefix.a2F.dat.

There is an example in PHonon/example/tetra_example/.