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



Dear Andrew, thank you for putting all this information together. The problem is really much more complex than a careless first look may suggest. Luckily I'm already using ORCA so I know that I have the right tool for the task.

Thank you again.

On Sun, Sep 10, 2017 at 11:51 AM, Andrew Rosen rosen###u.northwestern.edu <owner-chemistry.:.ccl.net> wrote:
Henrique,

I am going to preface this by saying that this is a deceivingly challenging task. I am a young graduate student who is very much still learning, so if others reading this disagree with something I say, please feel free to chime in. In general, you have the right idea, although there are a few things to consider. My response is in the context of DFT, which I imagine is what you're using.

1. I assume you are using the structure from XRD in your electronic structure calculations. It is likely that at the level of theory you choose, the XRD structure is not exactly the minimum energy structure (although it is hopefully quite close!). In that case, it may be advisable to do a geometry optimization from this experimental geometry instead of a single point energy calculation at each spin multiplicity. This may also be beneficial, albeit more computationally tasking, because different spin states can lead to different molecular geometries. Oftentimes, this difference may be small, but it has the possibility of impacting the energetics. The chapter on "Spin interactions in cluster chemistry" in the text "Advances in Inorganic Chemistry Volume 62: Theoretical and Computational Inorganic Chemistry" may be quite useful. A publicly accessible link to the relevant section on Google Books is found here. Pages 216-222 are extremely relevant to this discussion.

2. You should check to see the degree of spin contamination in your calculations, as discussed here. If it is large, it could signify that the level of theory you chose is not sufficient for the problem at hand and the energetics as well as other molecular properties may not be accurate. In such cases, it may be necessary to consider more accurate multireference methods.

3. It can often be difficult to accurately capture the relative energies of various spin states for transition metal complexes using DFT, and this can often be very sensitive to the choice of density functional. As discussed here, pure functionals tend to favor low-spin states whereas hybrid functions tend to favor high-spin states, and the energy difference between low- and high-spin states is often directly related to the amount of Hartree-Fock exchange in a given functional. This is a limitation to keep in mind.

4. There is the possibility that a broken-symmetry state is most stable. While this is likely a bit more involved than what you are looking to check for (especially given this is a trimer), it is worth realizing that such a possibility exists, as discussed here in the context of Gaussian or here in the context of ORCA.

5. At times, it may be necessary to check the stability of the wavefunction when dealing with open-shell structures. A detailed discussion on StackExchange can be found here.

In the end, yes, you are correct that you must do this "manually" and compare the energetics of different possible spin multiplicities. Your approach may end up being sufficient, but as I mentioned, there are some factors that you should at the very least keep in mind.

Andrew

On Sun, Sep 10, 2017 at 8:20 AM Henrique C. S. Junior henriquecsj+/-gmail.com <owner-chemistry^^ccl.net> wrote:

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


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Henrique C. S. Junior




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Henrique C. S. Junior
Industrial Chemist - UFRRJ
M. Sc. Inorganic Chemistry - UFRRJ
Data Processing Center - PMP