  We examine the quantum tunneling of magnetization in molecular spin in weak
interaction with a bath subject to Redfield master equation. By designing a
microscopic model for a multilevel spin system using only a generic Hamiltonian
and applying stationary approximation for excited doublets/singlets, we derive
a key equation of motion for the quantum tunneling of magnetization process
which is applicable in the whole temperature domain. From this equation, we
find that in general three tunneling rates are needed to accurately describe
the quantum tunneling process. More importantly, behavior of the quantum
tunneling in the intermediate temperature domain where there exists a
transition between incoherent and coherent quantum tunneling is also unraveled
for the first time. Limiting cases at low and high temperature and/or low
magnetic field are also worked out where some popular well-known results are
reproduced. Last but not least, a new interpretation of the quantum tunneling
of magnetization is proposed where we reveal the similarity between this
relaxation process with a driven damped harmonic oscillator.
