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["D. R. Hamann" <drhamann@mat-simresearch.com>]

Dear Souraya,

The content here can be changed depending on what you  want.  If you  only want elastic constants defined in the usual way and not the piezoelectric constants, you can drop both the "piezoflag 3"  line here and the entire dataset 2 (the d/dk calculation) in the input telast_2.in for the preceding abinit calculation.  You would also drop these for a metal, or an insulator whose piezoelectric constants were zero by symmetry.  For a system with one atom per unit cell, where there are no atomic-relaxation corrections, you should also drop the "rfphon" and "rfatpol" lines in the abinit run.  The anaddb run is only useful in this case to get the elastic constants in conventional units, and you can drop "instrflag" and set "elaflag  1."
 
That said, anaddb can also calculate the elastic constants under the boundary condition of zero electric displacement.  (The usual definition is zero electric field.)  There is usually only a very small difference, but the difference could be large for a material with very large piezoelectric constants.  For this calculation, you would need to keep the d/dk calculation in telast_2.in, and add "rfeld3 3" to the 3rd dataset in that file.  For the anaddb run, telast_3.in, you would have to set "elaflag 4" and "dielflag 3."

For a detailed explanation of this and what is going on in anaddb for the elastic constant calculation in general, you should read X. Wu et al.,  Phys. Rev, B 72, 035105 (2005).

Best regards,
Don Hamann

Souraya Goumri-Said wrote:

200903131000.n2DA09hQ003070@out2.fundp.ac.be" type="cite">Dear Abinitors,


I have a question about the the meaning of content of the file : telast_3.in  used to run anaddb in order to extract the elastic constants Cij

##########telast_3.in###########
!the input file for the anaddb code

  elaflag  3  !the flag for the elastic constant
piezoflag  3  !the flag for the piezoelectric constant
instrflag  1  !the flag for the internal strain tensor

!the effective charge part
   chneut  1  !enforce Born effective charge neutrality
# This line added when defaults were changed (v5.3) to keep the previous, old behaviour
  asr 0
  symdynmat 0


Must we change the reported values when we treat different systems ?

Best regards,

Cordialement,
Sincerely yours,

====================================
Dr. Souraya Goumri-Said
Centre de recherche en Physique de la Matière et du Rayonnement (PMR)
Laboratoire de Physique du Solide
Facultés Universitaires Notre-Dame de la Paix
Rue de Bruxelles 61 - B - 5000 Namur
Tel : +32(0)81 72 47 01
Fax : +32(0)81 72 47 10
http://sourayagoumri-said.weebly.com/
email : Souraya.Goumri-Said@fundp.ac.be

-- 
D. R. Hamann
Mat-Sim Research LLC    | Department of Physics
P.O. Box 742            |  and Astronomy
Murray Hill, NJ 07974   | Rutgers University
phone: 908-370-8079     | 732-445-5500, x4381

email: drhamann@mat-simresearch.com