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

[Stefano Baroni]

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.
>

...
>
> Thank you in advance for any help
> SATHYA SHEELA.S
> Grad. Student
> Department of Physics
> National Institute of Technology
> Tiruchirapalli - 620015
> India
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
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http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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