  An evidence of importance of the T-shaped configuration of coronene dimer is
presented. That is, the dimer's lowest energy configuration is not necessarily
a stack, as it might had been expected a priori. This is a surprising result
for dimer of such a large polycyclic aromatic hydrocarbon (PAH) as coronene.
The energy of the T-shaped configuration at all considered levels of density
functional theory (B3LYP,PBE/6-31+G(d),D95,cc-pVDZ,cc-pVTZ) was systematically
lower than the energies of three plausible stack configurations. In order to
get a better description of the van der Waals interaction,the density
functional theory (DFT) results were adjusted by adding a phenomenological
Lennard-Jones-type term into the total energy of the system. However, the van
der Waals correction is somewhat arbitrary and its magnitude can not be
rigorously justified. Depending on the choice of the parameters in the
phenomenological term both the T-shaped and the parallel-displaced (PD) stack
configurations can be the global energy minimum of the system. A simple model
is proposed which is useful for qualitative understanding of possible
geometries of the coronene dimer and larger coronene clusters. The model
represents coronene dimer as two sets of charged rings interacting via Coulomb
and Lennard-Jones potentials. The model provides an intuitively clear
explanation why the T-shaped dimers can be of importance even for some of
moderately large PAHs such as coronene and, may be, for circumcoronene.
