I am currently trying to determine how DFT+U values can be determined self consistently. I see that the bare and converged linear response matrix contain the difference in occupancies from the ground state. I am wondering if my understanding of the approach is correct.
- Calculate ground state and record the final occupancies
- Calculate perturbed state (bare) by applying U to the ground state charge density then taking the first set of occupancies from the first SCF step as the bare response matrix (bare - ground)
- Converge the bare state and take the final set of occupancies as the converged response matrix (converged - ground)
Then you end up with two matrices, the bare response matrix and the converged response matrix. The U value is then taken as follows.
U = (Converged-1) - (Bare-1)
My question is maybe naive, but how does this final step of subtracting two matrices give a U value.
I am using this as a resource to understand what is going on in this method.
If it at all matters, I would like to use CASTEP or GPAW as my calculator.