CCL: Correctly evaluating spin states of a cobalt trimer (using Single Points)?



Dear Henrique,

 

Excellent summary by Sergio. I have a couple of reinforcements and additions. 

 

Yes, you must optimize the geometry. Sometimes it's good practice to freeze one remote atom per ligand (not a hydrogen, and at least 3 bonds from any metal), to avoid large shifts in the ligand geometry. You must do this to identify the lowest spin state. You can then use single points on that geometry to check excitations to higher energy spin states, but the lowest state MUST be optimized. And to ensure you know which is lowest, you must optimize all states. 

 

Spin state energies are not completely reliable from any method. A better check if you have the correct one can be to compare geometries. Be aware if Xray problems though, below!

 

If you have a symmetric ligand sphere, but an unsymmetric wavefunction, it is quite possible that your observed Xray structure is an average of 3 isoergic structures. Xrays are generally not good enough to resolve this type of phenomenon, which can be caused by cocrystallization of similar geometries in different orientations, or by libration. Be aware if this when comparing geometries.

 

Make sure you have symmetry turned off, and/or that you check your final wavefunction for instability. 

 

BTW, I've said "wavefunction" here, even though I guess you're using DFT. If you can afford it, a high-level single point check might be good, for example using CCSD(T). In any case, you should check your results using some different DFT functionals, to see if your conclusions are sensitive to functional choice. Too many to chose from…

 

You may need to do hand-tweaking of the initial guess to achieve an antiferromagnetic result. This is an art, and very program dependent. I actually recommend Jaguar for that, very good keyword control of individual atomic spin state. But you CAN do it in most programs.

Finally, I would also recommend reading the article by Frank Neese, even though he misunderstood your oxidation state.

 

Best regards,

 

Per-Ola

Sent from my iPhone


On 11 Sep 2017, at 02:28, Sergio Emanuel Galembeck segalemb]![usp.br <owner-chemistry |-at-| ccl.net> wrote:

Dear Henrique,

 

I am not entirely sure if a single point is correct for study magnetic couplings. Normally, a geometry optimization is the first step for an

isolated system, because the experimental minimum is not the same as

a computational quantum chemistry one.

 

But, I think that your main concern is the broken symmetry problem and the multiconfigurational character of the wavefunction. 

 

Unfortunately, I don't work with these problems and techniques, but I can indicate

some Brazilian theoretical chemists that work with these subjects, or at least with multiconfigurational

methods. 

 

Best regards,

 

Sergio

 


Prof. Sergio Emanuel Galembeck

Computational Quantum Chemistry Laboratory

Departamento de Química - FFCLRP-USP

Av. Bandeirantes, 3900

14040-901 - Ribeirao Preto-SP

Brasil

 

phone: +55(16)33153765

 

2017-09-10 13:21 GMT-03:00 Henrique C. S. Junior henriquecsj,+,gmail.com <owner-chemistry###ccl.net>:

Dear Sergio, thanks for your reply. I forgot to tell that this structure is to be used to study magnetic couplings, so we usually don't perform geometry optimizations.

Despite that, do you believe that the Single Points approach is, in general, correct?

 

On Sun, Sep 10, 2017 at 11:07 AM, Sergio Emanuel Galembeck segalemb]-[usp.br <owner-chemistry__ccl.net> wrote:

Dear Henrique,

 

I suggest that for each spin state you optimize the geometry. Some of this states could generate an unstable geometry. 

 

Best regards,

 

Sergio


Prof. Sergio Emanuel Galembeck

Computational Quantum Chemistry Laboratory

Departamento de Química - FFCLRP-USP

14040-901 - Ribeirao Preto-SP

Brasil

 

 

2017-09-10 9:03 GMT-03:00 Henrique C. S. Junior henriquecsj+/-gmail.com <owner-chemistry~!~ccl.net>:

Dear colleagues, I’m working with a Cobalt(II) trimer whose molecular structure was achieved by Single Crystal X-Ray Diffraction. My task now is to check the spin states of the structure (High or Low spin). Since Co(II) can have 1 or 3 unpaired electrons, I’m approaching this problem by calculating Single Points for every possible multiplicity (10, 8, 6, 4, 2) and assuming that the most stable is the one that represents my structure (and my spin states).

Is this approach correct?

Thank you

 

--

Henrique C. S. Junior

 



 

--

Henrique C. S. Junior
Industrial Chemist - UFRRJ

M. Sc. Inorganic Chemistry - UFRRJ
Data Processing Center - PMP

Visite o Mundo Químico

 


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