  Dimerization of polycyclic aromatic hydrocarbons (PAHs) is an important, yet
poorly understood, step in the on-surface synthesis of graphene (nanoribbon),
soot formation, and growth of carbonaceous dust grains in the interstellar
medium (ISM). The on-surface synthesis of graphene and the growth of
carbonaceous dust grains in the ISM require the chemical dimerization in which
chemical bonds are formed between PAH monomers. An accurate and cheap method of
exploring structure rearrangements is needed to reveal the mechanism of
chemical dimerization on surfaces. This work has investigated the chemical
dimerization of two dehydrogenated PAHs (coronene and pentacene) on graphene
via an evolutionary algorithm augmented by machine learning surrogate
potentials and a set of customized structure operators. Different dimer
structures on surfaces have been successfully located by our structure search
methods. Their binding energies are within the experimental errors of
temperature programmed desorption measurements. The mechanism of coronene dimer
formation on graphene is further studied and discussed.
