[Pw_forum] negative value of polarization and Z* for Pb in Tetragonal PbTiO3 and discrepancy

[sathya subramanyan]

  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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