  Charge transport through molecular junctions is often described either as a
purely coherent or a purely classical phenomenon, and described using the
Landauer-B\"uttiker formalism or Marcus theory, respectively. Using a
generalised quantum master equation, we here derive an expression for current
through a molecular junction modelled as a single electronic level coupled to a
collection of thermalised vibrational modes. We demonstrate that the
aforementioned theoretical approaches can be viewed as two limiting cases of
this more general expression, and present a series of approximations of this
result valid at higher temperatures. We find that Marcus theory is often
insufficient in describing the molecular charge transport characteristics and
gives rise to a number of artefacts, especially at lower temperatures.
Alternative expressions, retaining its mathematical simplicity but rectifying
those shortcomings, are suggested. In particular, we show how lifetime
broadening can be consistently incorporated into Marcus theory, and we derive a
low-temperature correction to the semi-classical Marcus hopping rates. Our
results are applied to examples building on phenomenological as well as
microscopically-motivated electron-vibrational coupling. We expect them to be
particularly useful in experimental studies of charge transport through
single-molecule junctions as well as self-assembled monolayers.
