  Unitary transformations can allow one to study open quantum systems in
situations for which standard, weak-coupling type approximations are not valid.
We develop here an extension of the variational (polaron) transformation
approach to open system dynamics, which applies to arbitrarily large exciton
transport networks with local environments. After deriving a time-local master
equation in the transformed frame, we go on to compare the population dynamics
predicted using our technique with other established master equations. The
variational frame dynamics are found to agree with both weak coupling and full
polaron master equations in their respective regions of validity. In parameter
regimes considered difficult for these methods, the dynamics predicted by our
technique are found to interpolate between the two. The variational method thus
gives insight, across a broad range of parameters, into the competition between
coherent and incoherent processes in determining the dynamical behaviour of
energy transfer networks.
