[DFTB-Plus-User] Compute electrostatic potential with NEGF

Andrea Pedrielli andrea.pedrielli at unitn.it
Sun Oct 30 18:31:31 CET 2016

Thanks again,
essentially the aim of the work is to simulate the electron microscopy
holography response that is proportional to the integrated coulomb
potential along the beam direction. So, one electron goes through the
sample (in this case the nanotube) and feels a phase shift due to the
coulomb potential in each point along the path. The potential has to be
computed in each point of the space, also inside the atomic orbitals. In
the best case I would compute the potential from the charge density +
nuclei, like in DFT. I understand that the orbitals in DFTB are not changed
in shape. However, supposing a not so large change in the shape of the
orbitals it will be useful also only compute the potential starting from
the charge density of the non deformed orbitals. I tried to do this
plotting the the charge density using waveplot to a cube file and using an
Ewald summation over all the points. The problem is that the computational
cost is very high because I need a fine grid to describe the charge density
inside the atoms. If I understand well the Poisson solver doesn't take as
input the charge density that one can plot with waveplot but only the
difference of charge with respect to the reference configuration?


Il domenica 30 ottobre 2016, Gabriele Penazzi <penazzi at uni-bremen.de> ha

> Hi Andrea,
> I am not sure what you want to do, but dftb as a method may not include
> the details you need.
> I see now the point about mulliken charges from your previous mail, but
> the use of mulliken charges is pretty much a direct consequence of the way
> dftb (at least in its current and most popular version) is formulated.
> Nuclei+electron density does not make too much sense, as you work in a
> monopole approximation based on differences with respect to a reference
> configuration. The s-like projection of the potential reflects this and is
> consistent with the usual scc-dftb based on gamma-functional.
> For example, you have no potential in a system without self-consistent
> fluctuation. And even if you compare two system with different
> hybridization, if the scc component is zero the real space potential will
> be the same, i.e. zero. So my guess is no, you can not really have detailed
> information about he charge density between atoms and you probably have to
> go at DFT level.
> Best,
> Gabriele
> On 10/29/2016 11:55 PM, Andrea Pedrielli wrote:
> Thank you Gabriele,
> If I could, I would ask you another thing. I read in the online tutorial
> on silicon nanowire that the charge density is expanded in s spherical
> orbitals weighted with Mullen charges. Part of my interest in compute
> charge density and potential is take into account the charge density
> between the atoms, due to the bonds. If the charge density is expanded in
> s-like orbitals the charge density between the atoms (for example in a
> nanotube) is still well described?
> Andrea
> Il sabato 29 ottobre 2016, Gabriele Penazzi <penazzi at uni-bremen.de
> <javascript:_e(%7B%7D,'cvml','penazzi at uni-bremen.de');>> ha scritto:
>> Hi Andrea,
>> yes, you can do a non transport calculation with the real space Poisson
>> solver, I did the same for the same reason. Whether you can simulate your
>> system or not, however, may depend on the boundary conditions you want to
>> impose. For example, all periodic does not work if I remember right, but
>> for a 1D system you should be able to set up a well defined calculation.
>> Gabriele
>> On 10/28/2016 10:51 PM, Andrea Pedrielli wrote:
>> Hi users,
>> I need to compute the electrostatic potential produced by the charge
>> density from a dftb+ calculation. I know that there is no possibility using
>> dftb+, but I have seen that in the NEGF package there is a Poisson solver.
>> In particular I have to compute the inner and the outer potential of a
>> carbon nanotube, so a 1D object. Can I use a NEGF package for a
>> non-transport calculation with the aim of extract the electrostatic
>> potential? I underline that I need the potential due to the total charge
>> density+nuclei and Mulliken charges are not suitable for my purpose.
>> Thank you in advance,
>> Andrea
>> _______________________________________________
>> DFTB-Plus-User mailing listDFTB-Plus-User at mailman.zfn.uni-bremen.dehttps://mailman.zfn.uni-bremen.de/cgi-bin/mailman/listinfo/dftb-plus-user
>> --
>> Dr. Gabriele Penazzi
>> BCCMS - University of Bremen
>> http://www.bccms.uni-bremen.de/http://sites.google.com/site/gabrielepenazzi/
>> phone: +49 (0) 421 218 9328
>> fax: +49 (0) 421 218 4764
>> _______________________________________________
> DFTB-Plus-User mailing listDFTB-Plus-User at mailman.zfn.uni-bremen.de <javascript:_e(%7B%7D,'cvml','DFTB-Plus-User at mailman.zfn.uni-bremen.de');>https://mailman.zfn.uni-bremen.de/cgi-bin/mailman/listinfo/dftb-plus-user
> --
> Dr. Gabriele Penazzi
> BCCMS - University of Bremen
> http://www.bccms.uni-bremen.de/http://sites.google.com/site/gabrielepenazzi/
> phone: +49 (0) 421 218 9328
> fax: +49 (0) 421 218 4764
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