Question about TB2J Exchange and ligand downfolding

[Khaled Badawy]

Dear Xu He

I have Few questions about TB2J. I hope you can help me

1. Some materials exhibit AFM and FM ground states from DFT with different total energies. In cases the AFM state is not the ground state, TB2J yields positive J. Does that mean that the DFT AFM state is not robust against spin perturbations, or is not a local minimum? In other words, such a state would not be achievable in exp. given that there are no kinetic/external factors? 

2. Following with Q1, if one wants to determine the exchange interaction mechanism in a system where super- and direct- exchange mechanisms are competing, would the TB2J results give an idea by inspecting the J sign ? 

3. The metal-ligand downfolding method always returns the following error for version 0.9.9.14

python3.9/site-packages/TB2J/io_exchange/io_exchange.py", line 419, in get_full_Jtensor_for_Rlist
    Jmat[iR0][i, :, i, :] -= sum_JRi
IndexError: too many indices for array: array is 2-dimensional, but 4 were indexed

4. Are there upcoming magnon band structure for the AFM cases?

Thank you 

[Xu He]

Hello, 

- 1.  DFT can stablize both FM and AFM state. Then you can compare the energies of them to see which one is lower. (Although that does not necessarily mean the lower one is the ground state as there are other possible states).  The positive J indeed hints that the AFM is perhaps not the ground state. Sometimes it is not easy to know the ground state by just looking at a fist neighbour J, as the other J values can compete with it sometimes. A better way is to have a look at the magnon band structure.  (see https://www.nature.com/articles/s41524-024-01202-z). 

- 2. see answer to 1. 

- 3. Thanks for reporting. I will fix it today. 

-4. Yes. The new magnon module has been implemented and being tested. I think it will be available in this month. 

Best regards, 

HeXu

[Xu He]

Now the issue with the downfolding is fixed. 

Best regards, 

HeXu

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