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