I am running DFPT calculations for a tetragonal bulk perovskite structure (BTO) using the abinit executable. I obtain second and third order derivatives of the energy, as well as the dielectric tensor, Born effective charges, phonon frequencies and non-linear optical susceptibilities. All of the afore-mentionned quantities are finite.
However, when I post-process the derivative-database issued from the abinit executable with anaddb, I get infinity values for the same dielectric tensor and Born effective charges. The associated log file says:
The lowest mode appears to be a “true” negative mode,
and not an acoustic mode. This precludes the computation
of the frequency-dependent dielectric tensor.
Action : likely there is no action to be taken, although you,
could try to raise your convergence parameters (ecut and k-points).
For your information, here are the four lowest frequencies :
-5.61982526E-04 -5.61982526E-04 0.00000000E+00 0.00000000E+00
…” (frequencies in Hartree)
In summary, I get negative phonon frequencies (soft modes) for what seems to be the first 2 acoustic modes. I did increase ecut, the number of k-points and relaxed the structure even more but the two lowest frequencies remain negative regardless.
Why do the abinit and anaddb codes yield different results for the same quantities? I assume that the abinit output is more reliable than the anaddb output. How are the dielectric tensor and Born effective charges calculated from anaddb?
As a new user, I am not allowed to attach input/output files but I would be happy to do so if requested.
Master student at EPFL/ETH