Modeling Nonreactive Molecule−Surface Systems on Experimentally Relevant Time and Length Scales: Dynamics and Conductance of Polyfluorene on Au(111)

We propose a computationally efficient strategy to
accurately model nonreactive molecule−surface interactions that adapts
density functional theory calculations with the Tkatchenko−Scheffler
scheme for van der Waals interactions into a simple classical force field.
The resulting force field requires just two adjustable parameters per
atom type that are needed to capture short-range and polarization
interactions. The developed strategy allows for classical molecular
dynamics simulation of molecules on surfaces with the accuracy of highlevel
electronic structure methods but for system sizes (103 to 107
atoms) and timescales (picoseconds to microseconds) that go well
beyond what can be achieved with first-principles methods. Parameters
for H, sp2 C, and O on Au(111) are developed and employed to
atomistically model experiments that measure the conductance of a
single polyfluorene on Au(111) as a continuous function of its length.
The simulations qualitatively capture both the gross and fine features of the observed conductance decay during initial junction
elongation and lead to a revised atomistic understanding of the experiment.