Thanks for replying. But I have figured out the answer. I was looking for a simple QM treatment rather than just invoking Hund's rules. There was an excellent paper published by Antony Blake in J. Chem. Ed., Vol. 58, 1981, p393-398. It gives a very nice and simple QM rationalization for Hund's...
The additional stabilization from half-filled (e.g. 3d5, 4f7) and fully filled orbitals (e.g. 3d10, 4f14) is well-known. But can someone give an explanation for this? I'm looking for a theoretical rationalization. Thank you in advance.