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6:39:24 AM PST  Mon, Dec 10th 2018 

Hello,
When I compare the electron density difference (excited state minus ground state) of a RTTDDFT and a normal linear response TDDFT calculation they are quite different. For instance, given that the absorption spectra from both calculations are the same, if I do the RTTDDFT calculation, shouldn't the electron density at the maximum of the dipole oscillation be the same as the static TDDFT calculation?
Thanks,
Stacey




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5:31:20 AM PST  Thu, Dec 20th 2018 

Extracting the excited state density from a RTTDDFT calculation for a single electron excitation is problematic for a few reasons (e.g. https://aip.scitation.org/doi/10.1063/1.4900514). If you want to compare RTTDDFT and LRTDDFT results, your best bet is to calculate the transition density directly, which can be done via a Fourier transform of the density fluctuations in a RTTDDFT calculation (see https://aip.scitation.org/doi/10.1063/1.4742763 Eq. 20).
It can be a bit cumbersome to do and can potentially require a lot of disk space for all of the density matrices that need to be generated, but it will produce the same results as LRTDDFT, dependent on the quality of the Fourier transform. We used the same analysis in our paper on excited state absorption from RTTDDFT, where it worked beautifully (e.g. https://pubs.acs.org/doi/10.1021/acs.jctc.5b00473 Fig. 8, the agreement between LR and RT is not exact in this case for the reasons discussed in the paper).



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