[Pw_forum] Learning TDDFT calculation in Atomic label (CH4 molecule) using turbo_lanczos.x and turbo_davidson.x

[Timrov Iurii]

Dear Prem,


Please do not forget to write your affiliation.


> Dear All,
> I am a new user of QE and interested to calculate the optical absorption spectra in atomic level using > TDDFT code. I have checked the ecut & ecutfock convergence and also relaxed the structure. I can understand so far what is going on. But when I am running turbo_lanczos.x and turbo_davidson.x, I am getting output but how can I check that my calculation is correct ??

There is a reference data provided with the examples for the TDDFPT package.

You may compare your converged spectra with the experimental ones and with other theoretical spectra in the literature.

> After running turbo_davidson.x, I am getting a file named as "CH4.plot". If I am plotting it using xmgrace, it is showing the x-axis in Ry scale but that I want in eV ??

If you use turbo_davidson.x, plot the second column of CH4.plot (labeled as "total", oscillator strength S(E)) as a function of the first column (energy E in Ry). In xmgrace you can easily do a rescaling of the axes: in your case just make a conversion of the energy from Ry to eV by multiplying by 13.6.

If you use turbo_lanczos.x, you will obtain a file "CH4.plot_chi.dat". This file contains an information about the dipole polarizability tensor (chi_i_j) and about the oscillator strength S(E) which is: S(E) ~  E * sum_i Im[ chi_i_i(E) ]. Extract the oscillator strength "S(E)" from this file using the grep command, and then plot S(E) as a function of E which is the absorption spectrum.

The two codes (Lanczos and Davidson) must give you the same absorption spectrum (when both are converged). More details can be found in this paper: X. Ge et al., Computer Physics Communications, 185, 2080 (2014).

> Thanks,
> Prem

HTH

Regards,
Iurii

--
Dr. Iurii Timrov
Postdoctoral Researcher
Swiss Federal Institute of Technology Lausanne (EPFL)
Laboratory of Theory and Simulation of Materials (THEOS)
Lausanne, Switzerland

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