  We have calculated electron spin interactions in chains of Sc@C82 endohedral
fullerenes in isolation and inserted into a semiconducting or metallic
single-walled carbon nanotube to form a peapod. Using hybrid density functional
theory (DFT), we find that the spin resides mainly on the fullerene cage,
whether or not the fullerenes are in a nanotube. The spin interactions decay
exponentially with fullerene separation, and the system can be described by a
simple antiferromagnetic Heisenberg spin chain. A generalised Hubbard-Anderson
model gives an exchange parameter J and a Coulomb parameter U in good agreement
with the DFT values. Within the accuracy of the calculations, neither
semiconducting nor metallic nanotubes affect the interactions between the
fullerene electron spins.
