[Pw_forum] periodic boundary condition

[Axel Kohlmeyer]

On Sat, 2009-05-09 at 17:49 +0430, mohaddeseh abbasnejad wrote:
> Dear all,
>  
> I want to calculate the total energy of Si atom using CASINO without
> imposing periodic boundary conditions.
> At the first step, I should prepare a file containing wavefunction's
> cofficients by QE code.
> I know that for calculating the total energy of an atom, I have to use
> supercell, which it also imposes periodic boundary conditions.
> Would you please guide me how I should prepare the input file for QE
> such that it doesn't have any periodic boundary condition?

mohaddeseh,

Q-E uses plane waves as basis functions and those are by 
construction periodic. you can approximate the energy of
a single atom by systematically increasing the cell size
and extrapolating to infinity.

however, if you pay attention to the relative error vs.
infinite cell and compare it e.g., to the intrinsic error
of DFT or the error of using a finite basis set etc., then
you'll be able to determine a cell size where the error
from the periodicity (or limited cell size) is sufficiently
smaller than the others and thus it would not matter that
you have periodic boundaries.

that all being said, wouldn't it be easier to use a 
code that utilizes a localized basis set for a problem
like this? as little as i know about QMC code, they usually
support some of those as well.

cheers,
   axel.


> Thanks in advance.
>  
> Yours,
> Mohaddeseh
> 
>  
> 
> 
> -- 
> ---------------------------------------------------------
> 
> Mohaddeseh Abbasnejad, 
> Room No. 323, Department of Physics, 
> University of Tehran, North Karegar Ave.,
> Tehran, P.O. Box: 14395-547- IRAN 
> Tel. No.: +98 21 6111 8634  & Fax No.: +98 21 8800 4781
> Cellphone: +989177317514
> E-Mail:     m.abbasnejad at gmail.com
> Website:  http://physics.ut.ac.ir
> 
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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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If you make something idiot-proof, the universe creates a better idiot.