Modeling double-exchange in spin Hamiltonian

[ArunSridharan]

I'm modeling some magnetometry data collected on an [Fe4S4]2+ cluster using curry. The standard spin Hamiltonian used to model such systems includes terms for the usual Heisenberg exchange H = sum(J_ij S_i.S_j), as well as double-exchange terms for modeling resonance delocalization within parallel-aligned adjacent Fe(II)/Fe(III) centers that takes the form B_12(S_12 + 1/2) + B_34(S_34 + 1/2), where B_ij is a constant in energy units (value determined from least-squares fitting just as you would with J), and S_12 and S_34 are the subspins S_1+S_2 and S_3+S_4 (both are equal to 5/2 + 2 = 9/2 in this case).

Is there any way to include double-exchange terms of this form into the spin Hamiltonian used by Easyspin?

[Stefan Stoll]

EasySpin's spin Hamiltonian cannot handle such a mixture of terms in uncoupled and coupled representations. You could try to write the double-exchange terms in terms of the individual spin, and use the result to build Sys.