The potential perturbation caused by the displacement of a single atom is spatially localized. Hence, the phonon potential can be interpolated from q points on a coarse grid to other q points using Fourier interpolation. To use this functionality, first, one shoud run a ph.x calculation for q points on a regular coarse grid. Then, a dvscf_q2r.x run performs an inverse Fourier transformation of the phonon potentials from a q grid to a real-space supercell. Finally, by specifying ldvscf_interpolation=.true. in ph.x, the phonon potentials are Fourier transformed to given q points.
For insulators, the nonanalytic long-ranged dipole part of the potential needs to be subtracted and added before and after the interpolation, respectively. This treatment is activated by specifying do_long_range=.true. in the input files of dvscf_q2r.x and ph.x.
Due to numerical inaccuracies, the calculated Born effective charges may not add up to zero, violating the charge neutrality condition. This error may lead to nonphysical polar divergence of the phonon potential for q points close to Γ, even in IR-inactive materials. To avoid this problem, one can specify do_charge_neutral=.true. in the input files of dvscf_q2r.x and ph.x. Then, the phonon potentials and the Born effective charges are renormalized by enforcing the charge neutrality condition, following the scheme of S. Ponce et al, J. Chem. Phys. (2015).
The Fourier interpolation of phonon potential is proposed and described in the following papers:
A. Eiguren and C. Ambrosch-Draxl, Phys. Rev. B 78, 045124 (2008);
S. Ponce et al, J. Chem. Phys. 143, 102813 (2015);
X. Gonze et al, Comput. Phys. Commun., 248, 107042 (2020).