From this result, it is apparent that, given that E/D is much smaller than 1 by construction, the quantum spin tunnelling splitting becomes suppressed in the limit of large spin S, i.e., as we move from the atomic scale towards the macroscopic world. The magnitude of the quantum spin tunnelling splitting can be modulated by application of a magnetic field along the transverse hard axis direction (in the case of Hamiltonian [1], with D<0 and E>0, the x axis). The modulation of the quantum spin tunnelling splitting results in oscillations of its magnitude, including specific values of the transverse field at which the splitting vanishes. This accidental degeneracies are known as diabolic points.