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[889180013@s89.tku.edu.tw]

Hello,
   As my experience, the good pseudopotential is very important.
But I want to ask you is that, what is the force of your atoms?
You mentioned you use the experimental atomic coordinate.
It might undergo some force between atoms. Maybe you should
relax the atoms first. Then the harmonic approximation is applied.
Hope this will help you!

Yours truly,
Jerome

----- Original Message ----- 
From: <yukihiro_okuno@fujifilm.co.jp>
To: <forum@abinit.org>
Sent: Monday, January 31, 2005 10:59 AM
Subject: [abinit-forum] On the accuracy of the responce calculation (DFPT).


> Dear ABINIT Users.
>
> I'm now calculating the  piezoelectric coefficient , elastic constant
> of the PbTiO3,(PT) by Density functional perturbation theory, (DFPT).
>
> I found the Born effective charge are
> calculated reasonably good vaule, but
> the piezoelectric coefficient is far from the experiments and
> other calculate values ( PRL  vol(80) p4321).
>
> I'm informed in this mailing list that the reason is possibly due to
> the pseudopotential I used.
>
> I used Teter's extended norm conserved pseudo potential which is 
> downloaded
> from the ABINIT Home Page.   I also check  the TM norm conserved 
> potentials which are generated fhi98PP code. I check two
> TM norm conserved pseudo potential one of which is
> made by myself, and other is downloaded from the ABINIT web site.
>
> The value of poezoelectric tensor from the Teter's pseudo potential
> whici is down loaded from the web site,
>
> ================================================================================
>
> Calculation of the piezoelectric tensor
>
> -begin at tcpu      0.037   and twall      0.117sec
>
> Proper piezoelectric constants(clamped ion)(Unit:c/m^2)
>
>      0.00000000      0.00000000      0.33709523
>      0.00000000      0.00000000      0.33709521
>      0.00000000      0.00000000     -0.73010205
>      0.00000000      0.14564668      0.00000000
>      0.14564654      0.00000000      0.00000000
>      0.00000000      0.00000000     -0.00000002
>
> phonon modes warning- :
> accoustic 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:   -9.481483E-03    3.209976E-03   -9.481370E-03
>
> phonon modes warning- :
> unstable eigenvalue detected in force constant matrix at Gamma point
> the system under calculation is physically unstabale.
>
> Proper piezoelectric constants(relaxed ion)(Unit:c/m^2)
>
>      0.00001074      0.00001665      3.66885153
>      0.00001320      0.00001685      3.66884157
>      0.00002883      0.00007794     15.60848674
>      0.00000405     -2.89251432      0.00001611
>     -2.89249871      0.00000253      0.00001237
>      0.00000000      0.00000000      0.00000095
>
> ================================================================================
>
>
> The piezoelectric tensor which is calculated the pseudopotential which is 
> generated by Fhi98PP code. ( In this case , the ground state
> calculation is conserved by nband = 26, or iscf = 3)
>
> ================================================================================
>
> Calculation of the piezoelectric tensor
>
> -begin at tcpu      0.037   and twall      0.241sec
>
> Proper piezoelectric constants(clamped ion)(Unit:c/m^2)
>
>      0.00000000      0.00000000     -0.34997886
>      0.00000000      0.00000000     -0.34997886
>      0.00000000      0.00000000      0.82928878
>      0.00000000     -0.09788078      0.00000000
>     -0.09788078      0.00000000      0.00000000
>      0.00000000      0.00000000      0.00000000
>
> phonon modes warning- :
> accoustic 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:    9.096939E-04   -1.034723E-03    9.098799E-04
>
> Proper piezoelectric constants(relaxed ion)(Unit:c/m^2)
>
>      0.00001277      0.00001374     -1.79825769
>      0.00001491      0.00000232     -1.79824556
>      0.00001185      0.00004332     -4.39017143
>      0.00000854     -3.44122842      0.00000877
>     -3.44126548      0.00000690      0.00000169
>      0.00000000      0.00000000      0.00000000
>
> ================================================================================
>
> Results of these value are different each other
> and also both of results are different from the experimental values.
>
> I had checked the pseudo-potential of PbTiO3 by checking the each element 
> pseudo potential reproduce
> the lattice constants within the 1% from the experimantal values. ( Pb fcc 
> for Pb pseudo potential
> and TiC for Ti pseudo potential).
>
> Is the responce function calculation so sensitive to the pseudo potential 
> in general ? Or we must calculate more
> accurately in the step of ground state caluculation than usual
> ground state calculation?
>
> Sincerely,
>
> Yukihiro Okuno.
>
>
> My input file is below, ( I take the experimental atomic coorinate from 
> PRL vol(80) 4321 ).
>
> # Crystalline AlP - rhombohedral distortion imposed
> # Piezoelectroc tensor calculation
>
> ndtset  4
>
> #Common Data Definition of the unit cell
> acell       7.3730000000       7.3730000000       7.8522450000
> rprim     1.000000000000     0.000000000000     0.000000000000
>          0.000000000000     1.000000000000     0.000000000000
>          0.000000000000     0.000000000000     1.000000000000
> ntypat    3
> znucl 82 8 22
> natom    5
> typat 1 3 2 2 2
>
> xcart
>    0.000000000000     0.000000000000     0.000000000000
>    3.686500000000     3.686500000000     4.224507810000
>    3.686500000000     3.686500000000     0.918712665000
>    3.686500000000     0.000000000000     4.805573940000
>    0.000000000000     3.686500000000     4.805573940000
> ecut 50
> ixc 3
> ngkpt 4 4 4
> nband 26
> nbdbuf 0
> occopt 1
> nstep 500
> prtden 1
> prtvol 10
> tolwfr 1.0d-12
> diemac 12.0
>
> # First dataset : Self-consistent ground state run
> iscf1 5
> kptopt1 1
>
> # Second dataset : Non-self-consistent run for full k point set
> # we recover full 6 6 6 k space (not kptopt1 = 1)
> iscf2 -2
> kptopt2 3
> #use calculated results of dataset 1
> getden2 1
> getwfk2 1
>
> ## Third data set : finite-difference d/dk ground-state calculation
> berryopt3 -2
> getwfk3 2
> getden3 1
> iscf3 -2
> kptopt3 3
> # idir = 1 1 1 is needed because piezoelectric tensor need all d/dk 
> components
> rfdir3  1 1 1
>
> # Fourth dataset : electric field and strain responce
> getwfk4  2
> getddk4  3
> iscf4  3
> kptopt4 3
>
> #
> rfdir4  1 1 1
>
> # only generation of the first-order responce to the electric field
> # assuming that the data on derivative of ground state-wave function
> # with respect to k is available on disk
> rfelfd4  3
>
> # run strain responce function both uniaxial and shear strain
> rfstrs4  3
>
> # activate the calculation of the atomic dispacement
> rfphon4  1
>
> rfatpol4 1 5
>
> nqpt4 1
> qpt4 0.0 0.0 0.0
>
>
> # run phonon
>
>
> # strain perturbation is only available for v4.2 or over.
> # but still iscf = 2 or iscf = 3 and diemix <= 0.5 is reccomended
> diemix4  0.45
>
> diemac4  1.0
>
>