by **johnbrehm** » Thu Apr 19, 2018 9:22 pm

Thanks for your response.

Generally, I relax to a very small force, in this case :

max grad (force/stress) = 9.9119E-10 < tolmxf= 1.1000E-09 ha/bohr (free atoms)

Now, I am doing the R3mR phase of BaTiO3.

My relaxation yields the following which I carry all the way through to my phonon calc:

angdeg 8.97097924798673E+01 8.97087625785993E+01 8.97087950415756E+01

acell 7.69474233303469E+00 7.69372630864900E+00 7.69375849530342E+00

xred

-2.62152986528374E-03 -3.19046534588350E-03 -3.17144822412366E-03

4.83162648179077E-01 4.82631707831462E-01 4.82649520029828E-01

5.13368876766669E-01 5.12762392699501E-01 2.33746696455553E-02

2.40206560451921E-02 5.12743909756820E-01 5.12764100890483E-01

5.13369478890975E-01 2.33525070206667E-02 5.12783193516920E-01

Now I'm happy w/ my phonons: just 3 small neg acoustic (ie ok to assume stability and that they are == 0).

My freqs:

Phonon frequencies in cm-1 :

-3.122730E-02 -3.047196E-02 -2.813969E-02 1.540257E+02 1.540479E+02

1.572867E+02 2.587439E+02 2.591972E+02 2.649299E+02 2.884651E+02

2.884911E+02 3.072457E+02 4.568146E+02 4.568986E+02 5.329090E+02

Now, I am told my experimenters that the d33 for BTO is positive at ~100.

But I am getting a negative.

(the symmetries look ok, yes?)

Piezoelectric d tensor (relaxed ion) (unit:pc/N)

-96.67275336 42.33753525 42.06348693

41.86155809 -97.07601604 42.22293861

41.73607024 41.79187521 -97.16557179

12.81189036 -10.81879086 -12.15171696

-10.65680069 12.87695345 -12.30689921

-10.70041572 -12.17815700 12.86743783

Could the small negative inconsequential acoustic phonons be causing the issue?

I do get a warning sign right above my d tensor output:

ddb_piezo : WARNING -

Acoustic sum rule violation met : the eigenvalues of accoustic mode

are too large at Gamma point

Increase cutoff energy or k-points sampling.

The three eigenvalues are: 2.040229E-04 -6.580335E-04 -5.443626E-05

If so, how can I zero them out?

tnx,

JB