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## 4.2 Calculation of interatomic force constants in real space

First, dynamical matrices are calculated and saved for a suitable uniform grid of q-vectors (only those in the Irreducible Brillouin Zone of the crystal are needed). Although this can be done one q-vector at the time, a simpler procedure is to specify variable `ldisp=.true.` and to set variables `nq1`, `nq2`, `nq3` to some suitable Monkhorst-Pack grid, that will be automatically generated, centered at = 0.

Second, code `q2r.x` reads the dynamical matrices produced in the preceding step and Fourier-transform them, writing a file of Interatomic Force Constants in real space, up to a distance that depends on the size of the grid of q-vectors. Input documentation in the header of `PHonon/PH/q2r.f90`.

Program `matdyn.x` may be used to produce phonon modes and frequencies at any q using the Interatomic Force Constants file as input. Input documentation in the header of `PHonon/PH/matdyn.f90`.

See Example 02 for a complete calculation of phonon dispersions in AlAs.