Is it possible to do SAPT for two open shell S=1/2 fragments, which couple to a S=0 (closed shell) dimer?
No, Psi4’s SAPT is high-spin only.
Thanks so much for your prompt reply! Do you have me perhaps a pointer to a program package which implements also low spin?
No. A quick literature check gives me no hits for low-spin open-shell SAPT, not even a pilot code.
Hang on, there is Spin-Flip SAPT implemented by Konrad Patkowski which deals with general spin case SAPT: Symmetry-Adapted Perturbation Theory Isn’t it available? The reference is Konrad Patkowski, Piotr S. Zuchowski, Daniel G. A. Smith J. Chem. Phys. 148 , 164110 (2018).
Hi,
I had been wondering about this as well, and aside from the SF-SAPT mentioned above, I am also aware of the existence of a multiconfigurational SAPT being developed by Hapka, Przybytek and Pernal.
More information and the code (?) can be found in:
https://qchem.gitlab.io/gammcor-manual/pages/calculation/sapt_methods/index.html
I have not used it, so I do not know much more, but the papers looked interesting. I think the whole idea is based on the multiconfigurational description of the monomers, but I am not sure whether they can be coupled as different spins as well.
Best regards,
Carlos
ROHF-based SAPT in Psi4 can calculate the first-order exchange energy for an arbitrary spin coupling. This lowest-order spin-flip SAPT is documented at the link provided by @pzuch.
The multiconfigurational SAPT by Hapka and Pernal cannot yet do low-spin states of the complex, but some of us are working to change that 
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