Escape-Rate Crossover between Quantum and Classical Regimes in Molecular   Magnets: A Diagonalization Approach
  We have studied numerically the quantum-classical crossover in the escape-rate for an uniaxial spin system with an arbitrarily directed field. Using the simple quantum transition-state theory, we have obtained the boundary separating the first- and the second-order crossover and the escape-rate in the presence of the transverse and longitudinal field. The results apply to the molecular nanomagnet, ${\rm Mn_{12}}$. 
Gwang-Hee Kim (1) and E. M. Chudnovsky (2) ((1) Sejong University,   Korea, (2) City University of New York-Lehman College)