The electron transport mechanism changes from tunneling to hopping as molecular length increases. To validate the theoretical simulation after the transition point and clarify influence of electronic structures on the transition, we calculated the conductance of a series of conjugated molecules by density functional theory together with the nonequilibrium Green's function. We found that the highest occupied molecular orbital energy level, transmission spectrum, and the reorganization energy are good indicators for the transition of the electron transport mechanism. The calculated resistances of short junctions (<50 Å, before the transition point) are consistent with the experimental result, following the tunneling mechanism. However, the theoretical predication failed for long molecules, indicating the limitation of the theoretical framework of elastic scattering when the electron transport mechanism changes to hopping.
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