[gpaw-users] Calculating spectra and cataloging transition

[Michael Walter]

Dear M Nelson,

Am Do., 18. Juli 2019 um 11:32 Uhr schrieb Dr M C Nelson via gpaw-users <
gpaw-users at listserv.fysik.dtu.dk>:

> Hi
>
> I am trying to calculate and catalog fluorescent and phosphorescent
> spectra, and the associated transitions, for a set of organic and
> organometallic molecules each with 50 to 100 atoms.
>
> Is  LrTDDFT appropriate for this purpose?>
>

it depends what you want to know:
- transition frequency: you probably want to relax in the excited state
https://wiki.fysik.dtu.dk/gpaw/documentation/tddft/linear_response.html#relaxation-in-the-excited-state
- Rate: very difficult as you would have to find out all other channels of
relaxation (internal conversion)


> Is there a way to find out in an automatic way, how to specify the number
> of bands?
>

This number depends on the  density of states and up to which energy you
want to have your TDDFT transitions converged. Better use the keyword "
energy_range=" than istart and jend. Your calculation must provide a
sufficient number of unoccupied states and this number obviously depends on
the size of the molecule.


> In the tutorial for LrTDDFT,  the number of bands is specified in two
> different places, with two different values,
>
> From
> https://wiki.fysik.dtu.dk/gpaw/documentation/tddft/linear_response.html,
>
> # Create GPAW calculatorcalc = GPAW(nbands=10, h=0.3)# Attach calculator to atomsatoms.set_calculator(calc)
> # Calculate the ground stateenergy = atoms.get_potential_energy()
> # converge also the empty states (the density is converged already)calc.set(convergence={'bands': 8},
>          fixdensity=True,
>          eigensolver='cg')atoms.get_potential_energy()
> # Save the ground statecalc.write('Be_gs_8bands.gpw', 'all')
>
> And then a little farther on, we have
>
> c = GPAW('Be_gs_8bands.gpw')
> istart = 0  # band index of the first occ. band to considerjend = 8  # band index of the last unocc. band to considerlr = LrTDDFT(c, xc='LDA', istart=istart, jend=jend,
>              nspins=2)  # force the calculation of triplet excitations also
>
>
> Questions,
>
> in  calc = GPAW(),   why nbands = 10, and then later,  8?
>

There is no reason in this.


> Is there a away to determine how man bands to use in a general way, for
> example as a function of some parameter that can be retrieved from the
> Atoms object?
>

As a rule of thumb:  If you want to have "converged" excitation energies up
to energy E, include all Kohn-Sham transitions up to 2*E (energy_range=E)
might be fine. Find a bit more using istart, jend here:
http://dx.doi.org/10.1063/1.2943138

Regards,
Michael

>
>
> Thank you
> M Nelson
>
>
>
>
>
> _______________________________________________
> gpaw-users mailing list
> gpaw-users at listserv.fysik.dtu.dk
> https://listserv.fysik.dtu.dk/mailman/listinfo/gpaw-users



-- 
------------------------------------------
PD Dr Michael Walter
Addresses:
- Fraunhofer IWM, Wöhlerstrasse 11, D-79108 Freiburg i. Br., Germany
  Tel.: +49 761 5142 296
- FIT Freiburg Centre for Interactive Materials and Bioinspired
Technologies, Georges-Köhler-Allee 105, 79110 Freiburg, Germany
  Tel: +49 761 203 95072
email: Michael.Walter at fmf.uni-freiburg.de
www: http://www.functional-nanosystems.uni-freiburg.de/People/PDWalter/group
publications: http://scholar.google.com/citations?user=vlmryKEAAAAJ&hl=en
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listserv.fysik.dtu.dk/pipermail/gpaw-users/attachments/20190809/e17de98c/attachment.html>