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hehe

Member

Registered: 2025-10-30

Posts: 4

Advice on spin populations of spin-contaminated species

Hi Tian Lu,
                I would like to seek advice on a particular dilemma im having.

I am probing the electronic state/structure of a Ni cofactor that in literature is proposed to be 5 coordinate with a lower axial amino acid residue. (some suggestion is there for 6 coordinate with a potential weak coordination with the nature substrate(S)).

This Ni is proposed to be in the (I) state, which gives uncertainty on the 5 or 6 coordinate.

I was testing out several functionals for this system (annoyingly people were using B3LYP which im still using for comparsion)

I would like to discuss the use of the 4 functionals i would find pretty reliable here

PBE0-D4
R2-SCAN-D4 0 or 10 HF% exchange
WB97X-D4
TPSSH-D4

I have interestingly found that they converged to different geometries/wavefunctions

R2-SCAN and TPSSH gave a antiferromagnetically doublet with primariliy 2 alpha on Ni and a beta on the neighbouring carbon cofactor
Spin contamination is about ~ 1
Ni-S bond length of 2.48 (TPSSH) and 2.50 (R2-SCAN)
Mayer bond order of 0.7-0.8

PBE0 and WB97X gave a pure Ni(I) singlet that was more like 4 coordinate (or 5 - the below bond length feels a little too far even for a weak coordination)
Ni-S bond length of 3.07 (wB97XD) and 3.26 (PBE0)
Mayer bond order of 0.4-0.5 (I assumed this was a weak coordination but have been told to be careful in trusting this method)

From my understanding,
this is not a "problem" of overdelocalisation of DFT functionals (i.e self-interaction error) since the low or none HF exchange are giving a bond.
I am more afraid about the AF-doublet spin population i got from r2-scan and tpssh or im not sure if i can trust it?

What i have done now
1) re-Optimise r2-scan and tpssh with wb97x.gbw wavefunction
2) reoptimise tpssh with wb97x.gbw wavefunction
(I have tested stability of all wavefunctions with STABPerform true and they are all stable)

I wonder if the Mulliken spin populations might be unreliable and i should try a CASSCF single point to probe the spin populations on the R2-SCAN and TPSSH geometries.

I would like to seek advice on if my thought process thus far is fair
and any advice on which functional would be more reliable in this case or other methods i could use to probe the results thus far?

Thanks in advance ,
hehe

Last edited by hehe (2025-12-05 08:27:39)

sobereva

Tian Lu (Multiwfn developer)

From: Athens, Greece

Registered: 2017-09-11

Posts: 2,158

Website

Re: Advice on spin populations of spin-contaminated species

I would like to suggest using CASSCF to perform geometry optimization when some likely reliable DFT functionals gave very different geometries. Note that spin density cannot be evaluated for CASSCF wavefunction, but you can use Multiwfn to calculate odd electron density (OED) based on CASSCF wavefunction to characterize distribution of unpaired electrons, and you may compare it with |spin density| calculated by different functionals.

hehe

Member

Registered: 2025-10-30

Posts: 4

Re: Advice on spin populations of spin-contaminated species

Thanks for the correction on spin density. I know little about CASSCF and was doing some readings. I have seen your discussion of OED on other forums so was planning to follow up, depending on your response.
Thanks for the your feedback, i shall look into CASSCF

Kind regards,
hehe

Last edited by hehe (2025-12-06 10:43:58)