[Pw_forum] negative value of polarization and Z* for Pb in Tetragonal PbTiO3 and discrepancy
sathya subramanyan
sathya_sheela1985001 at rediffmail.com
Tue Jul 8 12:44:44 CEST 2008
Dear Sir,
I have checked the k -point sampling for the tetragonal PbTiO3 and I hope that the k-point sampling is alright. I am giving below the input file for your reference. Pl, let me know if anything is wrong in the input.:
THE I/P FILE FOR TETRAGONAL PbTiO3
&CONTROL
calculation = 'nscf'
pseudo_dir = '/home/nbaski/espresso-4.0/pseudo/'
outdir = '/home/nbaski/tmp/'
lberry = .true.
gdir = 3
nppstr = 7
/
&SYSTEM
ibrav = 6
celldm(1) = 7.3415
celldm(3) = 1.0653
nat = 5
ntyp = 3
nbnd = 22
ecutwfc = 30.0
occupations = 'fixed'
degauss = 0.00
/
&ELECTRONS
conv_thr = 1e-5
mixing_beta = 0.3
/
ATOMIC_SPECIES
Pb 207.2 Pb.vdb.UPF
Ti 47.867 Ti.vdb.UPF
O 15.9994 O.vdb.UPF
ATOMIC_POSITIONS
Pb 0.00000 0.00000 0.01000
Ti 0.50000 0.50000 0.53770
O 0.50000 0.50000 0.11180
O 0.00000 0.50000 0.61740
O 0.50000 0.00000 0.61740
K_POINTS {automatic}
4 4 7 1 1 1
Thank you in advance for any help
SATHYA SHEELA.S
Grad. Student
Department of Physics
National Institute of Technology
Tiruchirapalli
On Fri, 27 Jun 2008 Stefano Baroni wrote :
>the sum of the all the effective charges should be zero for any structure, due dynamical charge neutrality (or acoustic sum rule, or whatever you want to call the fact that a homogeneouse electric field will not exert a net (and infinite!) force on a crystal. Vioation of this charge neutrality/acoustic sum rule are often due to numerical inaccuracy, often inaccurate k-point sampling. you may want to check the convergence of your calculations with respect to the number of k points and/or try to calculate the effective charges using linear- response theory. SB
>
>On Jun 27, 2008, at 11:51 AM, sathya subramanyan wrote:
>
>>
>>Dear Developers,
>> I am trying to calculate the Berry Phaze Polarization and effective charge on Pb(with the displacement of . 01*ao) for PbTiO3(tetragonal) but am getting a negative value, using quantum espresso ver 4.0.
>>
>> When I run the example 10 for cubic phase the polarization and the Z* comes out to be alright. But, when I try for tetragonal phase, I am getting negative ionic phase leading to negative polarization for Pb.
>> If I compare the out put file for both cubic and tertragonal phase, to my surprise in the cubic phase the total ionic phase comes out to be positive (actually, if you sum it is negative), where as in the case of tetragonal phase the ionic phase comes out to be negative (but, it is positive if you sum the same way as for the cubic).
>> I do not understand this differrent summing rule for different phases. I am attaching both the input and out files for cubic and tetragonal phases. I would appreciate any useful suggestions to clear this point.
>>I am very sorry for the long mail and data files.
>>
>>- 620015
>>India
>>
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>
>---
>Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste
>http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
>
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>
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