Large-field versus discontinuous instantons in spin orientation   tunneling
  Tunnel splitting oscillations in magnetic molecules are reconsidered within the simplest model for the problem, which does not contain fourth order anisotropy. It is shown that at large magnetic field, there is only one instanton, and it is continuous. This is in contrast to the discontinuous instantons that are induced by the fourth order term [Ersin Ke\c{c}ecio\u{g}lu and A. Garg, Phys. Rev. Lett. {\bf 88}, 237205 (2002)]. 
Anupam Garg and Ersin Kececioglu