  In all theoretical treatments of electron transport through single molecules
between two metal electrodes, a clear distinction has to be made between a
coherent transport regime with a strong coupling throughout the junction and a
Coulomb blockade regime in which the molecule is only weakly coupled to both
leads. The former case where the tunnelling barrier is considered to be
delocalized across the system can be well described with common mean-field
techniques based on density functional theory (DFT), while the latter case with
its two distinct barriers localized at the interfaces usually requires a
multideterminant description. There is a third scenario with just one barrier
localized inside the molecule which we investigate here using a variety of
quantum-chemical methods by studying partial charge shifts in biphenyl radical
ions induced by an electric field at different angles to modulate the coupling
and thereby the barrier within the $\pi$-system. We find steps rounded off at
the edges in the charge versus field curves for weak and intermediate coupling,
whose accurate description requires a correct treatment of both exchange and
dynamical correlation effects is essential. We establish that DFT standard
functionals fail to reproduce this feature, while a long range corrected hybrid
functional fares much better, which makes it a reasonable choice for a proper
DFT-based transport description of such single barrier systems
