  Strong light-matter interactions facilitate not only emerging applications in
quantum and non-linear optics but also modifications of materials properties.
In particular the latter possibility has spurred the development of advanced
theoretical techniques that can accurately capture both quantum optical and
quantum chemical degrees of freedom. These methods are, however,
computationally very demanding, which limits their application range. Here, we
demonstrate that the optical spectra of nanoparticle-molecule assemblies,
including strong coupling effects, can be predicted with good accuracy using a
subsystem approach, in which the response functions of the different units are
coupled only at the dipolar level. We demonstrate this approach by comparison
with previous time-dependent density functional theory calculations for fully
coupled systems of Al nanoparticles and benzene molecules. While the present
study only considers few-particle systems, the approach can be readily extended
to much larger systems and to include explicit optical-cavity modes.
