[gpaw-users] Calculating spectra and cataloging transition

[Dr M C Nelson]

thank you

On 8/9/19 9:53 AM, Michael Walter wrote:
> 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 
> <mailto: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 calculator
>         calc  =  GPAW(nbands=10,  h=0.3)
>         # Attach calculator to atoms
>         atoms.set_calculator(calc)
>
>         # Calculate the ground state
>         energy  =  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 state
>         calc.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 consider
>         jend  =  8   # band index of the last unocc. band to consider
>         lr  =  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
>
>
>
>
>
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>
>
>
> -- 
> ------------------------------------------
> 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 
> <mailto: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

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