[DFTB-Plus-User] Applying an electric field with periodic boundaries

[Ben Hourahine]

Hello Ashwin,

you need to use something like:

Hamiltonian = DFTB {
  SCC = Yes
  ElectricField = {
    PointCharges = {
      CoordsAndCharges [AA] = {
        0.0 0.0  100.0  1.0 # Charge +1 at 100AA above layer
        0.0 0.0 -100.0 -1.0 # Charge -1 at 100AA below
      }
    }
  }

with the z direction of the supercell being large and the layer at z=0.

However, in the special case of a 2D graphene layer perpendicular to the
field the all of the atoms are at the same height, so experience the
same potential. The contribution added to the Hamiltonian in this form
of tight binding is proportional to the difference in potential between
atomic sites, so depending on which gauge is used the band structure
will shift up and down but not change. Similarly the gauge independent
energy won't change.

Regards

Ben

Ashwin Ramasubramaniam wrote:
> Hello Ben,
> 
> Thanks for your reply. I have a sheet of graphene which is periodic in x
> and y (and a large periodic distance in z) and I want to apply a uniform
> field across it in the z direction. The example in the autotest
> directory is not particularly clear to me. At any rate, is the idea that
> I model the electric field by inserting sheets of positive and negative
> charges (so that I basically have a parallel plate capacitor with
> graphene between the plates)?
> 
> thanks,
> -Ashwin.
> 
> 
> 
> 
>     On Wed, Aug 4, 2010 at 6:00 AM,
>     <dftb-plus-user-request at dftb-plus.info
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>           1. Re: Applying an electric field with periodic      boundaries
>              (Ben Hourahine)
> 
> 
>         ----------------------------------------------------------------------
> 
>         Message: 1
>         Date: Wed, 04 Aug 2010 10:05:44 +0100
>         From: Ben Hourahine <benjamin.hourahine at strath.ac.uk
>         <mailto:benjamin.hourahine at strath.ac.uk>>
>         Subject: Re: [DFTB-Plus-User] Applying an electric field with
>         periodic
>                boundaries
>         To: User list for DFTB+ related questions
>                <dftb-plus-user at dftb-plus.info
>         <mailto:dftb-plus-user at dftb-plus.info>>
>         Message-ID: <4C592D68.7000904 at strath.ac.uk
>         <mailto:4C592D68.7000904 at strath.ac.uk>>
>         Content-Type: text/plain; charset=ISO-8859-1
> 
>         Hello Ashwin,
> 
>         what type of electric field do you need to apply? Currently the code
>         can only include point charges when using periodic boundary
>         conditions.
>         The syntax is described in the  ElectricField section in the
>         dftb+ v1.1
>         manual, and there is an example in the autotest directory at:
>         scc/H2O-extchrg-periodic
> 
>         This is not equivalent to the recent methods to calculate properties
>         with constant values of E, D or P based on the modern theory of
>         polarization.
> 
>         Regards
> 
>         Ben
> 
>         Ashwin Ramasubramaniam wrote:
>         > Hello DFTB+ users,
>         >
>         > I see from the manual that an external field can only be
>         applied for
>         > non-periodic systems. Presumably a periodic system requires
>         the use of
>         > point charges. Is there an example available (or would any of
>         the users
>         > be kind enough to share one) for how to go about applying an
>         external
>         > field using point charges in the periodic case?
>         >
>         > thanks,
>         > -Ashwin.
>         >
> 
>         --
>             Dr. B. Hourahine, SUPA, Department of Physics,
>           University of Strathclyde, John Anderson Building,
>                  107 Rottenrow, Glasgow G4 0NG, UK.
>            +44 141 548 2325, benjamin.hourahine at strath.ac.uk
>         <mailto:benjamin.hourahine at strath.ac.uk>
>           The University of Strathclyde is a charitable body,
>         registered in Scotland, with registration number SC015263
> 
> 
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>         End of DFTB-Plus-User Digest, Vol 32, Issue 3
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> 

-- 
     Dr. B. Hourahine, SUPA, Department of Physics,
   University of Strathclyde, John Anderson Building,
          107 Rottenrow, Glasgow G4 0NG, UK.
    +44 141 548 2325, benjamin.hourahine at strath.ac.uk
   The University of Strathclyde is a charitable body,
registered in Scotland, with registration number SC015263