  Polaritonic chemistry has become a rapidly developing field within the last
few years. A multitude of experimental observations suggest that chemical
properties can be fundamentally altered and novel physical states appear when
matter is strongly coupled to resonant cavity modes, i.e. when hybrid
light-matter states emerge. Up until now, theoretical approaches to explain and
predict these observations were either limited to phenomenological quantum
optical models, suited to describe collective polaritonic effects, or
alternatively to ab initio approaches for small system sizes. The later methods
were particularly controversial since collective effects could not be
explicitly included due to the intrinsically low particle numbers, which are
computationally accessible. Here, we demonstrate for a nitrogen dimer chain of
variable size that any impurity present in a collectively coupled chemical
ensemble (e.g. temperature fluctuations or reaction process) induces local
modifications in the polaritonic system. From this we deduce that a novel dark
state is formed, whose local chemical properties are modified considerably at
the impurity due to the collectively coupled environment. Our simulations unify
theoretical predictions from quantum optical models (e.g. formation of
collective dark states and different polaritonic branches) with the single
molecule quantum chemical perspective, which relies on the (quantized)
redistribution of local charges. Moreover, our findings suggest that the
recently developed QEDFT method is suitable to access these locally scaling
polaritonic effects and it is a useful tool to better understand recent
experimental results and to even design novel experimental approaches. All of
which paves the way for many novel discoveries and applications in polaritonic
chemistry.
