  Plasmonic nanocavities enable the confinement of molecules and
electromagnetic fields within nano-metric volumes. As a consequence, the
molecules experience a remarkably strong interaction with the electromagnetic
field, to such an extent that the quantum states of the system become hybrids
between light and matter: polaritons. Here we present a non-perturbative method
to simulate the emerging properties of such polaritons: it combines a
high-level quantum chemical description of the molecule with a quantized
description of the localized surface plasmons in the nanocavity. We apply the
method to molecules of realistic complexity in a typical plasmonic nanocavity,
featuring also a subnanometric asperity (picocavity). Our results disclose the
effects of the mutual polarization and correlation of plasmons and molecular
excitations, disregarded so far. They also quantify to what extent the
molecular charge density can be manipulated by nanocavities, and stand as
benchmarks to guide the development of methods for molecular polaritonics.
