CCL: How to eliminate the spin contamination in the ORCA

 Sent to CCL by: Kathrin Helen Hopmann [kathrin.hopmann]_[uit.no]
 Dear Nikunj Kumar,
 Have you checked where the unpaired spin is located?
 2.71 sounds as if you may have a "broken symmetry triplet", which
 would imply excess beta spin in one place of the molecule (the ligand?) and
 excess alpha spin in another (for example a metal). If you have 1 unpaired beta
 spin on the ligand and 3 unpaired alpha spins on the metal you would get a
 broken symmetry triplet with a higher than expected S^2 value (the actual
 computed spin populations are probably smaller, for example 0.6 on ligand and
 2.6 on metal).
 In my experience, functionals with less (or no) HF exchange are more likely to
 give pure spin states (= less spin contamination). So if you are certain that
 the electronic structure should be a pure triplet, you could go for another
 (pure) functional and see what solution you get. If you obtain the pure triplet
 (2 unpaired alpha spins), you could try to also find the broken symmetry triplet
 with the other functional (this can be done by using the broken symmetry TPSSh
 electronic structure or the quintet electronic structure as input for the
 optimization). In that way you could get an idea if the pure or the broken
 symmetry triplet is lower in energy. It may be noted though that the energy of a
 broken symmetry state is approximate (you would have to do a spin projection of
 the energy to get a more correct value). You should probably also check the
 energy of the singlet as well as the quintet, just to be sure that none of these
 is lower than the triplet. And of course, as always, different functionals may
 give you different answers as to what is the preferred electronic state.
 best regards,
 Kathrin
 -----Original Message-----
 > From: owner-chemistry+kathrin.hopmann==uit.no . ccl.net
 <owner-chemistry+kathrin.hopmann==uit.no . ccl.net> On Behalf Of Nikunj
 Kumar nkk749 a gmail.com
 Sent: fredag 22. april 2022 11:38
 To: Kathrin Helen Hopmann <kathrin.hopmann . uit.no>
 Subject: CCL: How to eliminate the spin contamination in the ORCA
 Sent to CCL by: "Nikunj  Kumar" [nkk749..gmail.com] Dear all, I am
 working on an interesting catalyst system which has non-innocent ligand. I have
 optimized the triplet structure using the ORCA 4.2.1 employing the TPPSh
 functional. I was expecting the S^2 value to be around 2.0 but it came out to be
 2.71. Can we consider this as a triplet? Is this value reliable, if not how can
 we get rid of the spin contamination.
 Thanks
 Nikunj Kumarhttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txt