a1 Laboratoire de Chimie et Physique Quantiques, Université de Toulouse (UPS) and CNRS, IRSAMC, 118 route de Narbonne, 31062 Toulouse, France
a2 Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31062 Toulouse Cedex 4, France and CNRS, UMR 5187, 31028 Toulouse, France
a3 Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
a4 Université Montpellier II - GRAAL, CNRS - UMR 5024, Place Eugène Bataillon, 34095 Montpellier, France
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.
(Online publication March 30 2011)