Thanks for digging up the old threads. The 1/2 you found is the reduced cluster: one extra electron turns one iron into Fe(II) with S = 2, and 5/2 coupled anti

Thanks for digging up the old threads. The 1/2 you found is the reduced cluster: one extra electron turns one iron into Fe(II) with S = 2, and 5/2 coupled antiparallel to 2 leaves 1/2. This model is the oxidized form, two Fe(III), so the two 5/2 spins pair to 0. That's why the target is a singlet. Something I should have said in the first comment: the 35-second script prints S^2 = 3.50, not 1.3711, because it runs on a small test space. The 1.3711 is from the full-size calculation, and re-checking that takes about 11 hours on a 64-core machine. Both are far from 0, but if you ran the quick one expecting 1.3711 it would look like it didn't reproduce. Fair point on the blog post. I'm going to rewrite it now. The summary's the better link until then.