Re: About changing charge/multiplicity during QM-MM

From: Gerard Rowe (GerardR_at_usca.edu)
Date: Mon Apr 08 2019 - 11:26:48 CDT

If you have a single atom bridging your iron atoms at a shallow angle (>120 degrees), then the coupling between the iron centers is expected to be rather strong; probably in the hundreds of wavenumbers. A carboxylate can also act as a superexchange mediator, but it's not quite as good as a single mu-oxo.

There shouldn't really be anything special you need to do to converge the wavefunction of strongly coupled iron centers; it might even be easier due to the AFM state being so much more stable than the high-spin state. If you have experimental magnetic studies to compare your calculate J-value to, that's always nice, but with DFT, it's frequently considered a success if the sign of the number is correct. This is why I mentioned changing to something like B3LYP. I've worked on systems whose predicted J-value ranges from +20 cm-1 to -100 cm-1 just by screening a series of different functionals--_000_DM6PR19MB2987FFDB99E5C8F22E58CD73D12C0DM6PR19MB2987namp_--

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