[Smeagol-discuss] Problems in charge and convergence

[Stefano Sanvito]

Hi Wei,

> 1) In my calculations, the zero bias charges converged to 194.02 when 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