I am currently performing RASSCF calculations and would like to obtain the 1TT, 3TT, and 5TT states within a single output file. At present, I have only been able to achieve this by running three separate jobs with different values of the ras_spin_mult keyword. Could you please advise if there is a way to calculate all of these spin states within a single output file?
Additionally, I am using the crystal structure of an anthracene dimer, which shows three imaginary frequencies around the –30 cm⁻¹ region. Could you let me know whether these low-frequency modes are likely to cause any issues in the present calculations?
I’m not a RASSCF expert, but in general Q-Chem is not set up to compute different multiplicities within a single calculation except in some specialized cases (e.g., some versions of energy transfer couplings).
Regarding the imaginary frequencies, whether they matter depends on what you want to do with these calculations. If it’s just to get relative spin-state (electronic) energies then probably not, whereas if you’re trying to use a harmonic partition function to add an entropy correction and get free energy differences, then you need to worry about the fact that such a partition function is divergent for imaginary frequencies.
If you think they are artifacts, you can try re-optimizing the geometry with tighter stopping criteria. These keywords in $rem,
are 2x tighter than the default values, for example. That’s the easiest thing to try first; an alternative is to displace the geometry along the largest negative-curvature mode but there’s not an automated way to do that.
Thank you Sir, for your valuable feedback. I would like to clarify a point regarding my RASSCF calculations. Initially, I optimized my system in the quintet state, and in principle, from this step, I can get the energies of the 1TT, 3TT, and 5TT states. However, I am only obtaining the 1TT energy, while the 3TT and 5TT states are not appearing in the output though I am using a large number of excited state calculations.
Could you please advise me on how to proceed to obtain all three energy states? For your reference, I have attached the input file I am currently using: