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There is a possibility to evaluate rught estimation of UV-VIS absorption spectra by Fermi golden rule between Kohn-Sham orbitals. For given range range of photon energy are selected all pairs of occupied and unoccupied gorund-state Kohn-Sham orbital with appropriate energy difference. This two orbitals are than projected onto real space grid and transition dipole moment is computed. Oscilator strength (or probability of transition) is than proportional to squere of length of transition dipole moment. Absorbtion spectra can be than plotted by summing up contributions of oscillator strength for occupied-unoccupied orbital pairs with energy difference falling into particular energy bins (like weighted histogram).
Note that this method ( Fermi golden rule between ground state single particle states obtained from LDA-DFT ) is very crude approximation of true optical transitions. No beyond-DFT nor beyond-Hratree-Fock correction (such as RPA, TD-DFT, GW, BSE … ) are used. For this reason transition energy ( positioning of peak ) is mostly not realistic. The method is, however, useful to explore dependence of electronic transition probability on symmetry breaking and changes in shape and localization of electronic states.
Availability
this is implemented only in Prokop's personal versions of Fireball located in
/data/home/hapala/Fireball_dev/src_1.0-BSfinal /data/home/hapala/Fireball_dev/progs_Jellium_mod
Usage
The computation of transitions is activated by keyword iwrtexcit = 1 in firebal.in. However, currently it is also necessary to activate iwrtewf = 1 in order to initialize grid, even thought .xsf files written by iwrtewf option are not used at all and has nothing incommion with the computatuion of transitions ( this should be corrected in future). So the typical fireball.in looks like:
&OPTION basisfile = answer.bas nstepf = 1 icluster = 1 ifixcharge = 1 &END &OUTPUT iwrtewf = 1 iwrtexcit=1 &END &MESH iewform = 3 npbands = 1 pbands = 1 &END
