Dr. K. C. Bhamu píše v Út 15. 11. 2016 v 18:16 +0530: > Dear Prof. Peter > > This is in continuation with my previous query for excitation binding > energy and dielectric constant. > The first line of the case.epsilon is: > > # Energy [eV] Re_eps_xx Im_eps_xx Re_eps_zz Im_eps_zz > # > 0.013610 0.556574E+01 0.571125E-01 0.557539E+01 0.591910E-01 > > According to this data, the static dielectric constant comes out to > be: ~5.56. What does it mean? Should it be considered for low > frequency dielectric constant? > In our measured value we are getting low frequency dielectric > constant ~18 which is quite larger than calculated. We tried to > calculate low frequency dielectric constant using plazma frequency > and we got it 17.8 which is nearly equal to the measured value. For > this we used average band-gap value > > > In the literature I am getting low frequency dielectric constant > value between epslion_0 17-24 and at epscilon_infinite ~4-6 for > CH3NH3PbI3. > > I need any comments on it.
Dear Dr. K. C. Bhamu, to repeat what was said before, this is expected. What you get from the optic calculation (the case.epsilon) file is only the electronic part (and not even the whole electronic part) of the dielectric constant. This probably corresponds to the epsilon_infinity you are mentioning (but its hard to tell since epsilon_infinity usually stands for contributions to dielectric function from some higher energy processes but it does not say anything about what processes these are). As prof. Blaha said, to get the ionic part of the static dielectric constant you would need to do the BERRYPI calculations to get Born effective charges tensors. From my understanding (but I'm in no way an expert here, so hopefully someone can correct me if I mess something up) I think you would also need to do the phonon calculations and then you can combine it with the Born effective charges to get the mode- oscilator strengths and finally the ionic part of the static dielectric tensor, see eg. this article (first three equations) for some info: Rignanese, G.-M., Gonze, X. & Pasquarello, A. First-principles study of structural, electronic, dynamical, and dielectric properties of zircon. Phys. Rev. B 63, 104305 (2001). BTW if someone here at the mailing list did already calculated the ionic part of the static dielectric constant, the mode-oscilator strengths or anything related to IR optical absorption with Wien2k I would be interested to hear about it. > > Now using the average band-gap, we calculated " excitation binding > energy" which is now ~8 far below the experimental value 13. > This difference in the excitation binding energy may be due to no- > inclusion of the SO effect (or mBJ+SO as per your suggestion in last > email). The wise move would be to add the SO as was already advised by John McLeod and then deal with the band gap problem (scissor operator, mBJ or +U) to see it this improves your results. Best regards Pavel _______________________________________________ Wien mailing list Wien@zeus.theochem.tuwien.ac.at http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://email@example.com/index.html