  Electron transfer is an important and fundamental process in chemistry,
biology and physics, and has received significant attention in recent years.
Perhaps one of the most intriguing questions concerns with the realization of
the transitions between nonadiabatic and adiabatic regimes of electron
transfer, as the coupling (hybridization) energy, $J$, between the donor and
acceptor is varied. Here, using colloidal quantum dot molecules, a new class of
coupled quantum dot dimers, we computationally demonstrate how the
hybridization energy between the donor and acceptor quantum dots can be tuned
by simply changing the neck dimensions and/or the quantum dot size. This
provides a handle to tune the electron transfer from the nonadiabatic
over-damped Marcus regime to the coherent adiabatic regime in a single system,
without changing the reorganization energy, $\lambda$, or the typical phonon
frequency, $\omega_c$. We develop an atomistic model to account for several
donor and acceptor states and how they couple to the lattice vibrations, and
utilize the Ehrenfest mean-field mixed quantum-classical method to describe the
charge transfer dynamics as the nonadiabatic parameter, $\gamma$, is varied. We
find that charge transfer rates increase by several orders of magnitude as the
system is driven to the coherent, adiabatic limit, even at elevated
temperatures, and delineate the inter-dot and torsional acoustic modes that
couple most strongly to the charge transfer reaction coordinate.
