Incommensurate Transverse Anisotropy Induced by Disorder and   Spin-Orbit-Vibron Coupling in Mn12-acetate
  It has been shown within density-functional theory that in Mn$_{12}$-acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field. 
Kyungwha Park, Mark R. Pederson, Tunna Baruah, Noam Bernstein, Jens   Kortus, Steven L. Richardson, Enrique del Barco, Andre...