  Spin coherent state path integrals with discontinuous semiclassical paths are
investigated with special reference to a realistic model for the magnetic
degrees of freedom in the Fe8 molecular solid. It is shown that such paths are
essential to a proper understanding of the phenomenon of quenched spin
tunneling in these molecules. In the Fe8 problem, such paths are shown to arise
as soon as a fourth order anisotropy term in the energy is turned on, making
this term a singular perturbation from the semiclassical point of view. The
instanton approximation is shown to quantitatively explain the magnetic field
dependence of the tunnel splitting, as well as agree with general rules for the
number of quenching points allowed for a given value of spin. An accurate
approximate formula for the spacing between quenching points is derived.
