Keywords: user_guide

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## 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 λqν.
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 ωln 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/`.