I was reading on the the implementation of Hubbard 'U' in Density Functional Theory. The Hubbard 'U' can be thought of as a parameter that accounts for the on-site electron-electron interactions in correlated materials.
Meanwhile, the Hund's J, I'm given to believe is an exchange effect. Many texts mention the 'J' like this one. This kind of direct exchange comes from the antisymmetry nature of the many-body electron wavefunction. How can this kind of exchange be quantified by an energy value similar to the Hubbard 'U'? I believe in literature, this 'J' value is usually taken to be less than 20% of the U value. I have not found a conclusive reason for this, but what I am more concerned with, is how can you possibly quantify such a direct exchange?