UV/vis absorption via TDDFT
Posted: Mon Dec 24, 2012 9:44 pm
Dear Abinit Users and Developers!
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:
second and third are singlet and triplet excitations, like
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:
but first excitation energy is reported as ~ 15 eV:
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
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:
Code: Select all
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
Code: Select all
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:
Code: Select all
=== 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:
Code: Select all
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