Imagine a transition metal complex with multiple open-shell transition metal cations. If the metals are relatively close in distance, it is possible that antiferromagnetic coupling will occur between the metals. However, at approximately what distance does ferromagnetic vs. antiferromagnetic alignment of spins become isoenergetic due to negligible coupling between the metals? To provide some context, I'm asking about this in the context of density functional theory calculations. To make sure we're on the same page, let's call two magnetic configurations "isoenergetic" if the energies are, say, within ~1 kJ/mol or so.
The annoying answer is that this will be system dependent. I'll try my best to explain with a quick recap of magnetism.
- Unpaired electrons produce a magnetic moment
- One of the factors that determines the strength of the magnetic moment is the number of unpaired electrons.
If you have a specific system in mind, I would suggest performing a series of single-point (no ionic relaxations) calculations where you increase the distance between the ions. Contrast the energetic between FM and AFM configurations. If you have a license, VASP can provide information on the magnetic moments.