Please forgive my ignorance of experimental spectroscopy. I am a computational chemist, not an experimental one. My question is: what would be considered a tolerable or reasonable difference between an experimental lambda max from that taken from a CASSCF simulation? I am currently trying to design my active space, etc. My target is 4.56 eV but I can only get as close as 5.26 eV (+0.7 eV error) so far. Is it bad or should I proceed to my next step?

  • $\begingroup$ 0.7 eV is around 16 kcal/mol, i.e., quite a large error for many applications. ("Chemical accuracy" is normally defined as errors below 1 kcal/mol, i.e., <0.05 eV, but that may be too much for many applications too.) $\endgroup$ – Felipe S. S. Schneider Feb 7 '19 at 11:08
  • $\begingroup$ If your active space is indeed complete, then there would be nothing else you could do with CASSCF. It's a qualitative approach either way, so the numbers should point in the right direction, but you won't get them to match the experiment. I guess a thorough literature review would give you an idea what others do and consider working, and you might find out what is necessary to compare your results. Without knowing more of the context, it is hard to judge this error. $\endgroup$ – Martin - マーチン Feb 7 '19 at 11:14

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