Hello everyone, I would like to inquire whether it is possible to model antiferromagnetic coupling in QChem? Although it is described in this link (https://gaussian.com/afc/) that this can be achieved using Gaussian, I prefer to perform these calculations using QChem.
There are some tools for construction of effective (Heisenberg) Hamiltonians from EOM-CC calculations, see
https://manual.q-chem.com/6.0/Ch13.S6.SS1.html
However, if what you are looking to do is replicate what’s on the webpage that you mentioned (at SCF level of theory), that’s available too, it’s just a matter of specifying the appropriate multiplicity and generating the right guess. Sometimes finding the proper SCF solution for coupled transition metals can be tricky and there’s no single black-box prescription for how to do it. Tools like stability analysis are available in Q-Chem. You might consider SCF_GUESS=FRAGMO to generate fragments with proper individual spins that can be used to construct a superposition guess for the total density and spin.
In addition to EOM-CC/EOM-MP2, you can use SF-TDDFT to compute the couplings,
see, for example, here:
and RAS-CI, as described in the original work by Mayhall and Head-Gordon.
Dear John and Anna, Thank you both for your invaluable advice. Your answers are immensely helpful.