When we do a PES scan for chemical reactions with multiplicity change, for example, 2OH → H2O2, the multiplicity will change from 2 OH radicals to the product of H2O2. How may I set up the multiplicity for such reactions?
Thanks in advance!
But if your Q-Chem $molecule includes both OH radicals then you have to specify their spin coupling. What is the overall multiplicity for your 2 OH system? Is it singlet? In that case it seems like the whole reaction path is an overall singlet.
It depends which method do you use for these calculations. If you use something like EOM-CCSD or TDDFT, then the molecule section specifies the multiplicity of the reference state, and the target states are specified separately. For the example you described, I would suggest a spin-flip method with the triplet reference. You should be able to do such calculations either with EOM-SF-CCSD or with SF-TDDFT. This review paper
http://iopenshell.usc.edu/pubs/pdf/pccp-22-4326.pdf
describes the SF methods and also mentions studies in which crossings between different states were characterized.