[DFTB-Plus-User] DFTB-Plus-User Digest, Vol 77, Issue 13
Pu ZHANG
puzhang0702 at gmail.com
Mon Jun 23 17:00:14 CEST 2014
Dear Balint,
Thanks for your reply! I hope the development goes well :)
Best regards, Pu Zhang
--
Postdoc
The group of Structured Electromagnetic Materials
Department of Photonics Engineering
Technical University of Denmark
Ørsteds Plads, Building 345v, 166
2800, Kgs. Lyngby
E-mail: puzha at fotonik.dtu.dk
Homepage: http://www.dtu.dk/Service/Telefonbog/Person?id=91944&tab=0
Phone: 45 25 36 43
Fax: 45 93 65 81
On Mon, Jun 23, 2014 at 4:42 PM, <dftb-plus-user-request at dftb-plus.info>
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> Today's Topics:
>
> 1. Re: Has time-dependent DFTB been implemented in DFTB+?
> (B?lint Aradi)
> 2. Re: Correctness of SCC convergence in DFT+NEGF
> (anz118466 at cse.iitd.ac.in)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 23 Jun 2014 16:21:05 +0200
> From: B?lint Aradi <balint.aradi at bccms.uni-bremen.de>
> To: User list for DFTB+ related questions
> <dftb-plus-user at dftb-plus.info>
> Subject: Re: [DFTB-Plus-User] Has time-dependent DFTB been implemented
> in DFTB+?
> Message-ID: <53A837D1.6080007 at bccms.uni-bremen.de>
> Content-Type: text/plain; charset="utf-8"
>
> Dera Pu Zhang,
>
> > I would like to try to use this code to calculate optical spectrum. It
> > seems the time-dependent part has been under development. Has it been
> > released?
>
> No, it had not been officially released yet. We are in the process to
> improve the original formulation, in order to be qualitatively able to
> describe some features, the original formulation could not deliver. (See
> DOI: 10.1021/ct400123t). Unfortunately, forces are not implemented for
> the extension yet. :-(
>
> Best regards,
>
> B?lint
>
>
> --
> Dr. B?lint Aradi
> Bremen Center for Computational Materials Science, University of Bremen
> http://www.bccms.uni-bremen.de/cms/people/b_aradi/
>
>
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> ------------------------------
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> Message: 2
> Date: Mon, 23 Jun 2014 20:12:20 +0530
> From: anz118466 at cse.iitd.ac.in
> To: "User list for DFTB+ related questions"
> <dftb-plus-user at dftb-plus.info>
> Subject: Re: [DFTB-Plus-User] Correctness of SCC convergence in
> DFT+NEGF
> Message-ID:
> <76a5c978539a403ee59a49f0b39cd6bc.squirrel at webmail.iitd.ernet.in>
> Content-Type: text/plain; charset="utf-8"
>
>
>
> Hi Gabriele,
>
> For the second testcase, I had kept the poisson box as (30,30) Angstroms
> along the x and z directions. The transport direction is along y-axis and
> the 'MinimalGrid' was 0.2. These settings gave the weird convergence trend
> I had shared before.
>
> I repeated the simulation with a bigger poisson box (50,50) Angstroms
> along x and z axis, and a 'MinimalGrid' of 0.3. No convergence was
> achieved even after 700 iterations. In both the cases, the broyden
> 'MixingParameter' was kept as 0.3 and 'Temperature' as 300K. I am at a
> loss on how to proceed now.
>
> I am attaching the .gen file for your reference and also, the detailed.out
> for the (30,30) poisson box simulation. Kindly have a look. Any feedback
> would be very helpful.
>
> Thanks and Regards,
> Anusha
>
>
>
>
>
> > Hi Anusha,
> >
> > the Poisson box may indeed play a very important role in the convergence
> path.
> >
> > When everything is properly setup, equilibrium calculation are very
> often similar to a periodic dftb+ calculation, an SCC cicle of 14 steps
> is fine (a bit fast, even).
> > Hundreds of steps is definitely long and I would be suspicious as well.
> Also the SCC error gets very large.
> >
> > My experience is that a small Poisson box (along any direction) can be
> responsible for it. The mixer is quite robust, therefore I observed that
> with a small Poisson box you can still reach convergence to the exact
> solution but with very long and weird SCC steps. I suggest you to check
> that, in case of non-periodic calculation, you have enough vacuum around
> the device (if you can reach it computationally, a couple of nm on each
> side for each direction). The minimal grid step default is usually fine,
> at worst try to reduce it slightly (0.3).
> >
> > I rarely observed convergence to the trivial zero solution, i.e. all the
> electrons are shot away. I suggest you to check the detailed.out and
> verify that the charge population is reasonable.
> >
> > Hope this helps
> > Gabriele
> >
> >
> > On 06/20/2014 03:58 PM, anz118466 at cse.iitd.ac.in wrote:
> >> Dear all,
> >>
> >> I have simulated two testcases on the DFTB+NEGF. Both consist of a
> biomolecule on a GNR. I observed two very different trends in SCC
> convergence while simulating for the transmission spectra, which I am
> sharing here.
> >>
> >> The first case consists of 1795 atoms in total and saw an extremely
> fast convergence in just 14 steps. The second one, however, had 604
> atoms and saw convergence in around 600+ steps. Also, the SCC error in
> the second case increased gradually first and then decreased rapidly.
> For example, the SCC error reached a high of 0.21776430E+03 in 402
> steps. It then gradually decreased to 0.31423479E+02 in another 200
> steps, and fell sharply to 0.15680539E-05 in another 5 steps. The value
> of
> >> 'SCCTolerance'
> >> was kept 1e-4 in both the cases.
> >>
> >> Looking at these two trends, I am confused with the correctness of the
> final output. How do I ensure that no numerical noise has crept in any
> of
> >> my two testcases? Also, I observe that the final output changes by
> modifying the poisson box dimensions. Is there any rule of thumb
> regarding
> >> the choice of the poisson box? Please let me know if you need more data
> from me for further clarification.
> >>
> >> Thanks & Regards,
> >> Anusha Iyer
> >>
> >>
> >> _______________________________________________
> >> DFTB-Plus-User mailing list
> >> DFTB-Plus-User at dftb-plus.info
> >> http://www.dftb-plus.info/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 62337
> > fax: +49 (0) 421 218 62770
> >
> > _______________________________________________
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> > DFTB-Plus-User at dftb-plus.info
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> >
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