[Smeagol-discuss] Problems in charge and convergence

[Wei Fan]

I See.  I use only one K point in the other two direction in both the lead
and transport calculations as I have vacuum in those. Does it make sense if
I use more than one k point or it just helps converging?

Thanks in advance!
Wei

On Sun, Mar 23, 2008 at 5:41 AM, Stefano Sanvito <sanvitos at tcd.ie> wrote:

> Actually I meant periodic boundary conditions and k-points in the
> direction
> PERPENDICULAR to the the transport one, when you do the Smeagol
> self-consistent
> calculations (I am sure your leads are converged). Do you have periodic
> bounday
> conditions in that direction?
>
> Stefano
>
> Quoting Wei Fan <mail.fanwei at gmail.com>:
>
> > 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.Isthere
> > > 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
> >
>
>
> --
> 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
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