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Part of my work as an inorganic chemist is to investigate the magnetic coupling between metallic centers in coordination compounds. After some time, I've noticed that the classic PBE functional is the one giving me the best results (better than B3LYP, PBE0 or even M06) and, at first, I was convinced that this was a mere coincidence.

The cases in which PBE keeps giving me better results are adding up, so, I've decided to perform a quick test using the well known $\ce{[Mn(III)2O2(NH3)8]^{2+}}$ that have a reported experimental $J_{AB} = -186.5~cm^{-1}$.

My benchmark showed this:

To me, the results are that the Local and Gradient Corrected Functionals like PBE and LDA are, actually, better for this kind of properties in which orbital superposition and spin delocalization are important.

My point here, I think, is that I struggle to believe that such simple and "old" functionals like PBE and LDA are actually better describing a complicated phenomenon like magnetic coupling.

I'd love to hear what you, guys, think.

P. S. 1: I'm using ORCA, that gives me J calculated using 3 different formulas, that's why I have 3 J values.

P. S. 2: Sometimes I'm using 2 basis sets, a bigger one for Mn and a smaller for O, N and H.