Thanks, I think I fully understand the options now.
Best regards,
Ernesto.
On 21/10/16 09:38, Nick Papior wrote:
2016-10-21 9:05 GMT+02:00 Ernesto Ruiz <[email protected]
<mailto:[email protected]>>:
Dear SIESTA users,
I am currently trying to use the NetCharge and SimulateDoping
options (SIESTA 4.1), but I cannot understand some of the
explanations given in the manual. From what I can understand,
NetCharge sets a charge in the full volume of the supercell. Is
that right? Also, using this option for slabs is not recommended,
but it is used along with SimulateDoping in the sic-slab example.
What is the interaction between them? Is SimulateDoping used to
“correct” this extra charge?
NetCharge sets the charge of the system such that the Fermi-level is
determined so that the net-charge of the system is that set.
The NetCharge has "nothing" (see further below) to do with the
real-space grid, it is only used to determine the Fermi-level.
I.e. if NetCharge is 0 (the default). The Fermi-level is that of the
charge neutral system E_F(q=0). If NetCharge is q', the Fermi level
will be different such that:
\int_{E_F(q=q')}^E_F(q=0) DOS(e) de = q'
However, having a netcharge will introduce a charged cell (the
electronic charge and atomic charge will not cancel out) and this will
introduce an opposite equally spread charge in the unit-cell due to
the FFT solution of the Poisson equation. So in a way the NetCharge
also influences the full real-space grid, but this is related to the
Poisson solver.
Simulate doping takes the netcharge and puts it close to the atomic
cores (instead of the full cell) such that the Poisson solver "thinks"
it is a charge neutral cell with the doping close to the atoms.
Regarding Simulate Doping, it is stated that it “instructs the
program to add a background charge density to simulate doping”,
but the next line says that a compensating background charge is
used to make the system neutral. Is it the same background charge?
Does it take input from NetCharge to do so?
Yes, it takes the input from NetCharge. If NetCharge is zero, this
option does nothing.
My last question is related to one of the new options,
Geometry.Charge. For Geometry.Hartree, a potential is defined in a
region of a given shape but, how does Geometry.Charge introduce
this charge?
This is conceptually almost equivalent to NetCharge, you define a
region where you put a charge q, this charge is then also removed from
your system, i.e, the equivalent of NetCharge -q so that you still end
up with a charge neutral cell.
The Geometry.Charge may be used to put charge in vicinity of a system
to understand those effects. Basically you decide where to put the charge.
Also note that NetCharge and Geometry.Charge may be used together to
obtain a charged cell, if needed.
You may find additional information on this method in these papers:
DOI: 10.1039/C5CP04613K (method explained with gate-example)
DOI: 10.1016/j.cpc.2016.09.022 (method used on a simpler graphene system)
The Geometry.Hartree enables adding an additional term to the Hartree
potential in certain spatial regions.
Thank you for your help, I hope you can answer these questions.
Best regards,
Ernesto.
--
Kind regards Nick
