  EPR studies of a Ni4 single-molecule magnet yield the zero-field-splitting
(zfs) parameters, D, B40 and B44, based on a giant spin approximation (GSA)
with S = 4. Experiments on an isostructural Ni-doped Zn4 crystal establish the
Ni(II) ion zfs parameters. The 4th-order zfs parameters in the GSA arise from
the interplay between the Heisenberg interaction, Js1.s2, and the 2nd-order
single-ion anisotropy, giving rise to mixing of higher lying states into the S
= 4 state. Consequently, J directly influences the zfs in the ground state,
enabling its direct determination by EPR.
