I suggest checking what is the nature of the low-energy transitions (>1000 nm) using hole-electron analysis in Multiwfn, which can provide valuable physical insight.

Also, don't forget to confirm that the reference state wavefunction is stable.

I'm working on TD-DFT calculations of absorption/emission for fluorescein-type dyes (neutral and anionic forms), using wB97XD/Def2-TZVP with PCM.

For the neutral species, results match experiment very well. For the anionic species, however, I consistently obtain spurious low-lying excited states that do not match experiment, observing the appearance of some very low-energy transitions (>1000 nm) that are not seen experimentally. Since this is an anion, I first tried adding diffuse functions to fix it, but this did not resolve the issue.

I have managed to reproduce the experimental results very well by explicitly including the positive counterion, giving the overall system a net charge of zero. While this reproduces the results, I have doubts about this methodology, since upon optimization the Na+/K+ always ends up close to the negative charge, and I don't believe this is representative of the actual state in solution.

Do you think this methodology is acceptable? Any additional suggestions? Any guidance or relevant references would be greatly appreciated.

Thank you in advance, as always, for your outstanding work.

Best regards