Abstract
Models of surface enhancement of molecular electronic response
properties are challenging for two reasons: (1) molecule-surface
interactions require the simultaneous solution of the molecular and the
surface dynamic response (a daunting task); (2) when solving for the
electronic structure of the combined molecule+surface system, it is not
trivial to single out the particular physical effects responsible for
enhancement. To attack this problem, in this work we apply a formally
exact decomposition of the system’s response function into subsystem
contributions by employing subsystem DFT which grants access to dynamic
polarizabilities and optical spectra. In order to access information
about the interactions between the subsystems, we extend a previously
developed subsystem-based adiabatic connection fluctuation-dissipation
theorem of DFT to separate the additive from the nonadditive correlation
energy and identify the nonadditive correlation as the van der Waals
interactions. As an example, we choose benzene adsorbed on monolayer
MoS2. We isolate the contributions to the benzene’s
dynamic response arising from the interaction with the surface and for
the first time, we evaluate the enhancements to the effective
C6 coefficients as a function of
benzene-MoS2 distance and adsorption site. We also
quantify the spectral broadening of the benzene’s electronic excited
states due to their interaction with the surface. We find that the
broadening has a similar decay law with the molecule-surface distance as
the leading van der Waals interactions (i.e., R-6) and
that the surface enhancement of dispersion interactions between benzene
molecules is less than 5\%, but still large enough (0.5
kcal/mol) to likely play a role in the prediction of interface
morphologies.