Phenomenological modeling of molecular based rings beyond the strong exchange limit : bond alternation and single ion anisotropy effects

Non perturbative approaches, namely numerically exact diagonalization and quantum transfer matrix technique, are applied to Heisenberg spin systems to model antiferromagnetic ring shaped molecules. The Hamiltonian assumed includes the single ion anisotropy and alternating nearest neighbor exchange integrals. Using these techniques and exploiting the Hamiltonian symmetry group we have been able to perform numerically exact calculation beyond the strong exchange limit for relatively lage spin systems.