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

Electron transfer through a single barrier inside a molecule: from strong to weak coupling