The analysis of mid-IR emission suggests that a population of PAH-related very small
grains containing a few hundreds of atoms are present in the deep regions of molecular
clouds, although no specific species has been identified yet. In this review, we discuss
several candidates for these grains: neutral and ionised PAH clusters and complexes of
PAHs with Si atoms. The theoretical modelling of the properties of such molecular
complexes or nanograins is a challenging task. We first present an overview of quantum
chemistry derived models which can be efficiently used on-the-fly in extensive sampling of
the potential energy surfaces, as required by structural optimization, classical molecular
dynamics or Monte Carlo algorithms. From the simulations, various observables can be
determined, such as the binding energies, finite temperature IR spectra, nucleation and
evaporation rates. We discuss the relevance of those candidates in the molecular clouds
photodissociation regions and propose constrains and perspectives for the nature and size
of those very small grains.