  Tip-enhanced Raman scattering (TERS) has emerged as a powerful tool to obtain
subnanometer spatial resolution fingerprints of atomic motion. Theoretical
calculations that can simulate the Raman scattering process and provide an
unambiguous interpretation of TERS images often rely on crude approximations of
the local electric field. In this work, we present a novel and first
principles-based method to compute TERS images by combining Time-Dependent
Density Functional Theory (TD-DFT) and Density Functional Perturbation Theory
(DFPT) to calculate Raman cross sections with realistic local fields. We
present TERS results on the benzene and TCNE molecule, the latter of which is
adsorbed at Ag(110). We demonstrate that chemical effects on chemisorbed
molecules, often ignored in TERS simulations of medium and large systems sizes,
dramatically change TERS images. This calls for the inclusion of chemical
effects for predictive theory-experiment comparisons and understanding of
molecular motion at the nanoscale.
