I'm not quite sure what you're asking, but I'll try to answer. CASTEP is a first-principles program, so in general Hund's rules will be the result of a simulation, not an input. You can look at the projected atomic charges and spins at the end of a CASTEP calculation.

If you're thinking more of J in a "DFT+U" Hubbard-like context, then this doesn't actually appear in most formulations. Dudarev et al. show that the effects of J actually cancel out between the terms, to leading order:

https://doi.org/10.1103/PhysRevMaterials.3.083802

so only the Hubbard U term is required.

Thank you;

I saw in the paper (Orbital-selective behavior in cubanite CuFe2S3. DOI: 10.1103/PhysRevB.105.024406 ) on CuFe2S3 cubanite that the authors chose Jh = 0.95 eV and U = 8 eV for Cu and U = 6 eV for Fe; using the VASP code. That is why I asked the question.

thank you.

This probably means they used U = 8 - 0.95 = 7.05 eV, sometimes called "U effective", as their Hubbard U. The paper I linked to by Dudarev et al. suggests that in fact this should just be "U", although using U=7.05 eV might be fine.

Dudarev shows that if you start from a proper Stoner Hamiltonian and derive the Hubbard terms in "DFT+U", J should not appear.

Merci beaucoup.