  Tip-enhanced Raman spectroscopy (TERS) is a valuable method for surface
analysis with nanometer to angstrom-scale resolution, however, the accurate
simulation of particular TERS signals remains a computational challenge. We
present a unique approach to this challenge by combining the two main
contributors to plasmon-enhanced Raman spectroscopy and to the high resolution
in TERS in particular, the electromagnetic and the chemical effect, into one
quantum mechanical simulation. The electromagnetic effect describes the
sample's interaction with the strong, highly localized and inhomogeneous
electric fields associated with the plasmonic tip, and is typically the
thematic focus for most mechanistic studies. On the other hand, the chemical
effect covers the different responses to the extremely close-range and highly
position-sensitive chemical interaction between the apex tip atom(s) and the
sample, and, as we could show in previous works, plays an often underestimated
role. Starting from a (time-dependent) density functional theory description of
the chemical model system, comprised of a tin(II) phthalocyanine (SnPc) sample
molecule and a single silver atom as tip, we introduce the electromagnetic
effect through a series of point charges that recreate the electric field in
the vicinity of the plasmonic Ag nanoparticle. By scanning the tip over the
molecule along a 3D grid, we can investigate the system's Raman response on
each position for non-resonant and resonant illumination. Simulating both
effects on their own already hints at the achievable signal enhancement and
resolution, but the combination of both creates even stronger evidence that
TERS is capable of resolving sub-molecular features.
