[Smeagol-discuss] Problems in charge and convergence

[Stefano Sanvito]

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.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
>


--
Prof. Stefano Sanvito
School of Physics and CRANN,
Trinity College Dublin, IRELAND
Tel: +353-1-8963065
www.smeagol.tcd.ie