  A large number of non-adiabatic dynamical studies have been applied to reveal
the nature of carrier transport in organic semiconductors with different
approximations. We present here a "nearly exact" graphical process unit (GPU)
based finite temperature TD-DMRG method to evaluate the carrier mobility in
organic semiconductors as described by electron-phonon model, in particular to
rubrene crystal, one of the prototypical organic semiconductors, with
parameters derived from first-principles. We find that (i) TD-DMRG is a general
and robust method that can bridge the gap between hopping and band picture
covering a wide range of electronic coupling strength; and (ii) with realistic
parameters, TD-DMRG is able to account for the experimentally observed
"band-like" transport behavior ($\partial \mu / \partial T < 0$) in rubrene. We
further study the long-standing puzzle of isotope effect for charge transport
and unambiguously demonstrate that the negative isotope effect ($\partial \mu /
\partial m < 0$ where $m$ is the atomic mass) should be universal.
