Single-ion anisotropy effects on the energy spectra of spin S = 1 Heisenberg ring

The energy spectra of the quantum spin S = 1 ring in the presence and absence of magnetic field are calculated. The numerical exact diagonalization technique exploiting the point-group symmetry is applied to the Heisenberg spin Hamiltonian with single-ion anisotropy modelling approximately the dodecanuclear nickel cluster Ni12(O2CMe)12(chp)12(H2O)6(THF)6. The energy-level dependence of the anisotropy parameter D is analyzed and the field-dependent level-crossing is presented.