[Smeagol-discuss] Problems in charge and convergence

[Wei Fan]

Dear  Stefano,

It is a good idea to increase the screening lead part in the scattering
region, I will try to test it. For your questions, I have used 13 Al atoms
to compose the buffering atoms inside the scattering region, which is a ABC
structure along [111].  For the bulk leads,  I  have used periodic boundary
conditions along transport direction [111]. And I have used 100 k points
along this direction.  I doubt 2 or 4 k points is enough or not as  one
usually needs Fermi level of the leads accurate enough.

Wei

On Sat, Mar 22, 2008 at 1:16 PM, Stefano Sanvito <sanvitos at tcd.ie> wrote:

> Hi,
>
> OK, you probably can increase NEnergImCircle to 50-60 although probably it
> will
> not help here. I believe the problem is with the fact you are trying to
> use 1D
> leads. In these screening is really poor and you need huge unit cells to
> converge. How many atoms of the leads you have in your scattering region?
> A
> better strategy would be to use some more bulky leads, better if you use
> some
> periodic boundary conditions. You usually don't need many k-points (2 or 4
> should be enough for Al).
>
> Stefano
>
>
> Quoting Wei Fan <mail.fanwei at gmail.com>:
>
> > Dear  Stefano,
> >
> > Thank you very much for your suggestions. In my calculations,  the
> > parameters for charge integration are
> >
> > NEnergReal          857
> > NEnergImCircle       20
> > NEnergImLine         20
> > NPoles               10
> >
> > I have tried to increase NEnergReal to 2000, but it helped a little.
> Then
> > when I increase MaxScfSteps to 800,  the charges finally converged after
> > really a lot of steps. Although it takes much, it works anyway.Is there
> a
> > possible way to accelerate the convergence for higher bias?
> >
> > Thanks in advance!
> > Wei
> >
> > On Sat, Mar 22, 2008 at 5:59 AM, Stefano Sanvito <sanvitos at tcd.ie>
> wrote:
> >
> > > Hi Wei,
> > >
> > > > 1) In my calculations, the zero bias charges converged to 194.02when it
> > > is
> > > > 194.00 in the first scf step. I am not sure if it's acceptable.
> However,
> > > the
> > > > weird thing is that the converged charge gets bigger when bias is
> > > larger,
> > > > 194.03 when bias=0.1V, 194.037 when bias=0.2V,  194.044 when bias=
> > > 0.3V......
> > > > Does it make sense or is there something wrong?  Btw, The system
> > > consists
> > > > of  two wire Al leads along 111 with a 1,4-benzene-dithiolate in the
> > > > center.  The system is recognized as a Chain in siesta.
> > >
> > > In general there is nothing particularly wrong with that. The
> > > electrostatics is
> > > different at different bias and it is not surprising that the
> > > self-consistent
> > > charge density in the scattering region changes with bias. Whether
> this is
> > > correct for the specific system you are looking at, it is difficult to
> > > say.
> > > Usually problems arise when the charge density systematically
> decreases
> > > with
> > > bias (by many electrons). This is not the case here, so this looks
> > > approximately OK.
> > >
> > > > 2) Generally, I can't get converged densities when bias is above 1
> volt
> > > with
> > > > a 10-4 tolerance. The charge difference in those biases with zero
> bias
> > > is
> > > > more than 0.1.  Is there something responsible for this?
> > >
> > > There might be several reason for that. Did you try to change the
> number
> > > of
> > > energy points along the real axis? You might not have too many and the
> > > charge
> > > density is integrated on a too course grid. How many are you using?
> > >
> > > Regards,
> > >
> > > Stefano
> > > --
> > > Prof. Stefano Sanvito
> > > School of Physics and CRANN,
> > > Trinity College Dublin, IRELAND
> > > Tel: +353-1-8963065
> > > www.smeagol.tcd.ie
> > >
> > >
> >
> >
> > --
> > Wei Fan
> > Room 332, Main Building
> > Nano-Organic Photoelectronic Laboratory
> > Technical Institute of Physics and Chemistry
> > Beijing, China, 100101
> > Tel: +86-13811270970
> > +86-10-82680161
> >
>
>
> --
> Prof. Stefano Sanvito
> School of Physics and CRANN,
> Trinity College Dublin, IRELAND
> Tel: +353-1-8963065
> www.smeagol.tcd.ie
>
>


-- 
Wei Fan
Room 332, Main Building
Nano-Organic Photoelectronic Laboratory
Technical Institute of Physics and Chemistry
Beijing, China, 100101
Tel: +86-13811270970
+86-10-82680161
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.tchpc.tcd.ie/pipermail/smeagol-discuss/attachments/20080322/6de2ad97/attachment.html