I'm very curious in the calculation of optical absorption spectra of single molecule using TDDFT, and faced with a couple of questions.
I started with TDDFT because it looks simpler then GW, and its limitations (finite system, not a metal) should not yield any harm at current stage of research.
Unfortunately in order to calculate absorption TDDFT tutorial redirects to the paper, not available for me.
As I understood, once you have oscillator strengths, the only thing left to do is to smear them with appropriate Lorentzian function. Is it right?
Here comes more questions:
1) Abinit calculates three types of oscillator strengths:
first type:
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Kohn-Sham energy differences, corresponding total energies and oscillator strengths (X,Y,Z and average)-(oscillator strengths smaller than 1.e-6 are set to zero)
Transition (Ha) and (eV) Tot. Ene. (Ha) Aver XX YY ZZ
15-> 16 1.87230E-01 5.09480E+00 -3.75501E+01 2.7008E+00 8.10E+00 0.00E+00 0.00E+00 ...second and third are singlet and triplet excitations, like
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TDDFT singlet excitation energies (at most 20 of them are printed), and corresponding total energies.
Excit# (Ha) and (eV) total energy (Ha) major contributions
1 1.93857E-01 5.27513E+00 -3.754344E+01 0.51( 15-> 16) 0.49( 14-> 17) ...They all differ each other! Here is example for the case of benzene (eV):
Which oscillator strengths should be used for UV/vis absorption?
2) Comparison with available experimental data gave even more mess. To clarify this I've tried single atom in a big box. Hydrogen calculation failed due to 0 fully occupied bands, so I have used Helium, and have got absolutely weird results..
Helium absorption starts from ~2 eV, and Abinit correctly shows this in log file as:
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=== Gap info ===
>>>> For spin 1
Minimum optical gap = 1.2432 [eV], located at k-point : 0.0000 0.0000 0.0000
Fundamental gap = 1.2432 [eV], Top of valence bands at : 0.0000 0.0000 0.0000
Bottom of conduction at : 0.0000 0.0000 0.0000but first excitation energy is reported as ~ 15 eV:
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Transition (Ha) and (eV) Tot. Ene. (Ha) Aver XX YY ZZ
1-> 2 5.54893E-01 1.50994E+01 -2.31227E+00 0.0000E+00 0.00E+00 0.00E+00 0.00E+00 which also could be seen at this picture.
What is wrong here? Should these two values (2 eV and 15 eV) be coincide?
Thank you for any clarifications,
Leon