# Frequently Asked Questions

- What type of Coulomb repulsion uses the code? How do I check if the
form is what I think it is?

**Answer**: Here is a note on the Coulomb repulsion, which should help.
cdmft

- What is the exact double-counting, and how is it implemented?

**Answer**: The exact double-counting can be derived from the
DFT+DMFT Luttinger-Ward functional. If one applies both the LDA and
the DMFT approximation on the exact Luttinger-Ward functional, one
obtains the intersection, which is the exact double-counting. This was
derived in PRL 115, 196403 (2015).

To understand this point better, here is a
very short presentation
with some equations.

Finally, to implement the exact double-counting, one needs to solve
the electron-gas problem, which interacts with the screened Coulomb
repulsion. The best would be to solve this with Monte Carlo, but in
the absence of MC results, the GW results can be used as a reasonable
approximation (but only for the ration between the unscreened and
screened correlation). The results of such a GW calculations,
can be found here: