Hi all,

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:

“— !ERROR

src_file: m_ddb_diel.F90

src_line: 184

mpi_rank: 0

message: |

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.

Many thanks

Master student at EPFL/ETH