Intermolecular Interactions in Molecular Tunnel Junctions: Significant and Still Compatible with the Picture of Single Molecule Junctions in Parallel

Intermolecular van der Waals (vdW) interactions are inherently present in molecular tunnel junctions fabricated with self-assembled monolayers (SAMs), but characterizing their effects is challenging; current state-of-the-art theory in molecular electronics lacks a dedicated property tailored for this purpose. Based on an analysis that goes beyond the traditional framework and is able to accurately reproduce <i>I-V</i> curves measured on benchmark molecular junctions, we show here that the half-width Λ of the molecular orbital (MO) mediating charge transport is a suitable property for quantifying these vdW effects. Our results demonstrate that vdW interactions prevail over MO-electrode interactions in determining Λ. This contradicts a common claim that the MO width originates from MO-electrode interactions. The present theoretical framework also allows us to solve a conundrum of molecular electronics, namely how and why, despite significant intra-SAM interactions, it is legitimate to describe molecular junctions formed by a bundle of molecules as <i>independent</i> single-molecule junctions in parallel.