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Vibrational frequencies, relaxed-ion static dielectric tensor
and relaxed-ion piezoelectric tensor

Setting `IBRION`=8 or `IBRION`=7 selects the calculation
of the interatomic force constants using density functional perturbation theory
(see also Sec. 6.22.7),
whereas `IBRION`=6 or `IBRION`=5 uses finite displacements to
determine the interatomic force constants (see also Sec. 6.22.6).
For `IBRION`=8 and `IBRION`=6, symmetry is taken into account, whereas
the other setting neglect symmetry considerations and are thus significantly
slower.

If `IBRION`=5-8 is selected *and* `LEPSILON`=.TRUE. is selected,
the relaxed-ion static dielectric tensor, or low frequency dielectric tensor,
as well as the relaxed-ion piezoelectric tensors
are determined [110].
All values are collected
and printed at the end of the OUTCAR file.
Specifically the ionic contribution to the piezoelectric tensor is printed after

PIEZOELECTRIC TENSOR IONIC CONTR for field in x, y, z (C/m^2)

and the ionic contributions to the dielectric tensor are printed after:
MACROSCOPIC STATIC DIELECTRIC TENSOR IONIC CONTRIBUTION

Note that `LRPA`=.FALSE. (default) must be selected to obtain these values.
For hybrid functionals, vasp presently does not support linear response calculations.
However, one can set `LCALCEPS=.TRUE.` and `IBRION`=6 in the INCAR file
to calculate the Born effective charges by applying a finite field
and the interatomic force constants using finite differences.
The ionic contribution to the piezoelectric tensor and the dielectric tensor
are then calculated as well.

N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at