Hello everyone, I'm carrying out a broken-symmetry calculations in a V, Cr and Ti complexes and for V and Cr DFT works well but in the case of Ti complexes it doesnt recognize the multiplicity that I introduce into the input, because the calculation take me to a point where there is not any spin density in the Ti atom. (I'm following this procedure https://gaussian.com/afc/)
The broken-symmetry situation is between the central Ti atom and the pyrazine ligands, wich are radicals. I really appreciate any comment.
That is an issue which goes down to the most basic fundamentals of DFT. The Hohenberg-Kohn theorems which are the initial theorems of DFT to my extent of knowledge are only proven for non-degenerate ground states which is no longer the case if you go to higher multiplicities.
Any multiplicity which goes beyond a singlet consists of multiple states, e.g. a triplet consists of |up up>, |down down> and 1/sqrt2 (|up down>+|down up>).
DFT as a single-reference quantum chemistry method can only grab one out of these three, so if you demand a triplet, it will just create the |up up> state and ignore the others. Therefore, if you get a reasonable result out a simple triplet DFT calculation, that's more luck than systematic science.
In order to include such effects more properly, you can either resort to higher-quality wavefunction methods which are of course more expensive or you can use modified DFT with effective hamiltonians. The latter methods were used by my former colleagues at KIT, see e.g.
Article A litmus test for the balanced description of dispersion int...
and references therein. These were done with Turbomole, though, I don't know if Gaussian offers these methods.
Jürgen Weippert thanks for your answer, but the problem is that in the case of my Cr complex DFT reproduces well the spin density, but in the case of the analogous complex with Ti it fails and gives a spin density of 0.0002 (when i expect something around 1)
Is it a Ti4+ complex? In that case your result might actually make sense. What is your multiplicity and is there significant spin density within the pyrazine?
Jürgen Weippert No, it is a Ti3+ complex (TiCl2(pyz)4) and there is one electron delocalized among the pyrazines. It is weird, because in the analogous CrCl2(pyz)4 DFT works well.
The multiplicity of the system should be 1: one electron in Ti and another among the pyrazines (antiferromagnetic)
OK, that sounds like the SCF might have ended up creating sort of a "fake charge transfer" in which an electron from the Ti gets transferred to the ligands thus creating the illusion of a closed shell.
You could check the occupation of the orbitals manually, put an electron from a ligand into an orbital on the Ti and restart the optimization.
By the way, from where do you know that the oxidation states are like that? Do you have EPR or something like it to verify it?
Jürgen Weippert I obtained the optimize structure from VASP (because the real material is a MOF) and I am using a fragment of this structure to calcule the J between. Because of that I am doing single-point calculations on the high-spin and broken-symmetry case, but it's there when I observe this wrong spin density.
I know the oxidation states because this systems were studied in this paper: https://www.nature.com/articles/s41467-022-33342-5
Thank you very much for the help
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