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Re^2: Geometry optimization problem with MCSCF with ECP in C3 symmetry.

Alex Nest

On Thu Aug 11 '16 11:51pm, Pedro Silva wrote
>I do not think you should force symmetry or use ROHF for Fe-S clusters, due to the presence of anti-ferromagnetic coupling between the two up spins in two Fe3+ ions and the down spin on the third Fe3+.
>I have posted detailed suggestions on

Thank you for advice, Pedro.

Reading your material I understand that symmetry can be broken in some oxidative states of Fe-S clusters. So I simply removed the symmetry by NOSYM=1 and my cluster started to converge both in CASSCF(3,3) and ROHF-MP2.

Intrestingly, resulting geometries become Cs symmetry and are very similar in both cases (attached).

By the way, I understand that you do not recommend "symmetry" and you do not recommend "ROHF". But WHAT do you recommend?

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[ casscf_vs_rohfmp2.png ]

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