  The polarizable embedding (PE) approach is a flexible embedding model where a
pre-selected region out of a larger system is described quantum mechanically
while the interaction with the surrounding environment is modeled through an
effective operator. This effective operator represents the environment by
atom-centered multipoles and polarizabilities derived from quantum mechanical
calculations on (fragments of) the environment. Thereby, the polarization of
the environment is explicitly accounted for. Here, we present the coupling of
the PE approach with the density matrix renormalization group (DMRG). This
PE-DMRG method is particularly suitable for embedded subsystems that feature a
dense manifold of frontier orbitals which requires large active spaces.
Recovering such static electron-correlation effects in multiconfigurational
electronic structure problems, while accounting for both electrostatics and
polarization of a surrounding environment, allows us to describe strongly
correlated electronic structures in complex molecular environments. We
investigate various embedding potentials for the well-studied first excited
state of water with active spaces that correspond to a full
configuration-interaction treatment. Moreover, we study the environment effect
on the first excited state of a retinylidene Schiff base within a
channelrhodopsin protein. For this system, we also investigate the effect of
dynamical correlation included through short-range density functional theory.
