Effective Hamiltonians from Spin Adapted Configuration Interaction

A generalised extraction procedure for magnetic interactions using effective Hamiltonians is presented that is applicable to systems with more than two sites featuring local spins $S_i\ge 1$. To this end, closed, non-recursive expressions pertaining to chains of arbitrary equal spins are derived with the graphical method of angular momentum. The method is illustrated by extracting magnetic couplings from ab initio calculations on a [CaMn${}_3$${}^{(IV)}$O${}_4$] cubane. An extension to non-sequential coupling schemes proves conducive to expressing additional symmetries of certain spin Hamiltonians.