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

Dear Mzahgan,

Let's keep this discussion on the forum so others may benefit.

Your input file looks good in a quick scan.  A few things to look for in 
your output file are whether the first-order wave function calculations 
converged within your 100-iteration limit, and whether there were 
non-zero terms in the list of internal strain parameters at the end.  
The issue here is that I believe the cubic structure of PbTiO3 is 
unstable relative to a ferroelectric distortion.  If there are non-zero 
internal strain terms that means that atomic relaxation makes 
contributions to the elastic constants.  However if there are unstable 
phonon modes, trying to calculate the relaxed-atom elastic constants 
using anaddb will fail, indicating (probably) that the cubic structure 
is unstable with respect to one or more strain distortions.  I don't 
have access to the Liu paper at the moment, so I don't know what they 
did, but if the problem is ill-defined, it's unclear what the comparison 
of results means.

Don Hamann

Mozhgan Amini wrote:
> Hi,
>  
> I have done the "Elastic properties" tutorial, and here is my full 
> input file.
> Thanks a lot in advance.
> #PbTiO3
> #Response function calculation for:
> #    * rigid-atom elastic tensor
> #    * rigid-atom piezoelectric tensor
> #    * interatomic force constants at gamma
> #    * Born effective charges
>    ndtset   3
> # Set 1 : Initial self-consistent run
>     iscf1   5
>   kptopt1   1
>   tolvrs1   1.0d-18  #need excellent convergence of GS quantities for 
> RF runs
>
> # Set 2 : Calculate the ddk wf's - needed for piezoelectric tensor and
> #         Born effective charges in dataset 3
>   getwfk2  -1
>     iscf2  -3        #this option is needed for ddk
>   kptopt2   2        #use time-reversal symmetry only for k points
>     nqpt2   1        #one wave vector will be specified
>      qpt2   0 0 0    #need to specify gamma point
>   rfelfd2   2        #set for ddk wf's only
>    rfdir2   1 1 1    #full set of directions needed
>   tolwfr2   1.0d-20  #only wf convergence can be monitored here
> # Set 3 : response-function calculations for all needed perturbations
>   getddk3  -1
>   getwfk3  -2
>     iscf3   5
>   kptopt3   2        #use time-reversal symmetry only for k points
>     nqpt3   1
>      qpt3   0 0 0
>   rfphon3   1        #do atomic displacement perturbation
>  rfatpol3   1 5      #do for all atoms
>   rfstrs3   3        #do strain perturbation
>    rfdir3   1 1 1    #the full set of directions is needed
>   tolvrs3   1.0d-10  #need reasonable convergence of 1st-order quantities
> #Common input data
>
> # acell  COPY RELAXED RESULT FROM PREVIOUS CALCULATION
> # Here is a default value, for automatic testing : suppress it and 
> fill the previous line
>   acell  3*3.863 angstrom
>  
>   rprim   1.0  0.0  0.0   #hexagonal primitive vectors must be
>           0.0  1.0  0.0   #specified with high accuracy to be
>           0.0  0.0  1.0   #sure that the symmetry is recognized
>                           #and preserved in the optimization
>                           #process
> #Definition of the atom types and atoms
>  ntypat   3
>   znucl   82 8 22
>   natom   5
>   typat   1 3 2 2 2
> #Starting approximation for atomic positions in REDUCED coordinates
> #based on ideal tetrahedral bond angles
> # xred  COPY RELAXED RESULT FROM PREVIOUS CALCULATION
> # Here is a set of default values, for automatic testing : suppress it 
> and fill the previous line
>   xred  0.0    0.0    0.0
>         0.5    0.5    0.5
>         0.5    0.5    0.0
>         0.5    0.0    0.5
>         0.0    0.5    0.5
>
> #Gives the number of bands, explicitely (do not take the default)
>   nband   13             # For an insulator (if described correctly as an
>                          # insulator by DFT), conduction bands should not
>                          # be included in response-function calculations
> #Definition of the plane wave basis set
>    ecut   45.0            # Maximum kinetic energy cutoff (Hartree)
>  ecutsm   0.5            # Smoothing energy needed for lattice paramete
>                          # optimization.  This will be retained for
>                          # consistency throughout.
> #Definition of the k-point grid
>  kptopt   1              # Use symmetry and treat only inequivalent points
>   ngkpt   6 6 6          # 4x4x4 Monkhorst-Pack grid
> nshiftk   1              # Use one copy of grid only (default)
>  shiftk   0.5 0.5 0.5    # This choice of origin for the k point grid
>                          # preserves the hexagonal symmetry of the grid,
>                          # which would be broken by the default choice.
> #Definition of the self-consistency procedure
>  diemac   6.0            # Model dielectric preconditioner
>    iscf   5              # Use conjugate-gradient SCF cycle
>   nstep   100            # Maxiumum number of SCF iterations
>
>
> --- On *Wed, 9/17/08, D. R. Hamann /<drhamann@mat-simresearch.com>/* 
> wrote:
>
>     From: D. R. Hamann <drhamann@mat-simresearch.com>
>     Subject: Re: [abinit-forum] calculation of elastic constants for
>     PbTiO3
>     To: forum@abinit.org
>     Date: Wednesday, September 17, 2008, 5:40 AM
>
>     Dear mozhganamini,
>
>     Your input file does not look like an elastic-constants input file.  
>     Have you done the "Elastic properties" tutorial?  Without the full
>     input 
>     file that gave these results, it is hard to diagnose your problem.  I 
>     see no "tol..." input variables, and the convergence criteria to get 
>     good elastic constant (and other response function) results are much 
>     tighter than the defaults that your attached ground-state file 
>     apparently would get.
>
>     Don Hamann
>
>     mozhganamini@yahoo.com wrote:
>     > Dear all,
>     > I'm trying to calculate the elastic constants of PbTiO3 for comparing
>     the
>     > results with:  " Y. Liu et al. , Materials Science and Engineering A
>     472(2008)
>     > "
>     > but my results are approximately twice the results in this article!?
>     > Thanks a lot in advance
>     >
>     > my results for elastic           results in article
>     > C11: 629 (GPa)                              383
>     > C12: 192					 151 
>     > C44: 116					 120
>     >
>     > my input file:
>     >
>     >  acell 3*3.863 angstrom
>     >  rprim 1.0  0.0  0.0 
>     > 	   0.0	1.0  0.0   
>     > 	   0.0	0.0  1.0	  
>     > #Definition of the atom types and atoms
>     >  ntypat   3 
>     >  znucl	 82 8 22
>     >  natom  5
>     >  typat  1 3 2 2 2
>     >
>     >   xred 0.0    0.0    0.0
>     >        0.5    0.5    0.5
>     >        0.5    0.5    0.0
>     >        0.5    0.0    0.5
>     >        0.0    0.5    0.5
>     >
>     >  nband	 13	 
>     > 	      
>     > #Definition of the plane wave basis set
>     >    ecut   45.0	 
>     >  ecutsm   0.5	     
>     > 			 
>     > #Definition of the k-point grid
>     >  kptopt   1	     
>     > ngkpt	6 6 6	
>     > nshiftk   1	       
>     >  shiftk   0.5 0.5 0.5	
>     > 	   
>     > #Definition of the self-consistency procedure
>     >  diemac   6.0	 
>     >    iscf   5		
>     >   nstep   100	      
>     >
>     >   
>
>     -- 
>     D. R. Hamann
>     Mat-Sim Research LLC    | Deptartment of Physics
>     P.O. Box 742            |  and Astronomy
>     Murray Hill, NJ 07974   | Rutgers University
>     phone: 908-370-8079     | 732-445-4381
>
>     email: drhamann@mat-simresearch.com
>
>
>               

--
D. R. Hamann
Mat-Sim Research LLC    | Deptartment of Physics
P.O. Box 742            |  and Astronomy
Murray Hill, NJ 07974   | Rutgers University
phone: 908-370-8079     | 732-445-4381

email: drhamann@mat-simresearch.com