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Electronic excited states of benzene in interaction with water clusters : influence of structure and size

This work is dedicated to the theoretical investigation of the influence of water clusters' organisation and size on the electronic spectrum of an interacting benzene (Bz) molecule using both TD-DFT and CASPT2 approaches. Two series of geometries, namely $Geo_{IEI}$ and $Geo_{IED}$ were extracted from two Bz-hexagonal ice configurations leading to maximum and minimum ionization energies respectively. An appropriate basis set containing atomic diffuse and polarisation orbitals and describing the Rydberg states of Bz was determined. The TD-DFT approach was carefully benchmarked against CASPT2 results for the smallest systems.Despite some discrepancies, the trends were found to be similar at both levels of theory: the positions and intensities of the main $π\rightarrow π^{\star}$ transitions were found slightly split due to symmetry breaking. For the smallest systems, our results clearly show the dependence of the electronic transitions on the clusters' structures. Of particular interest, low energy transitions of non negligible oscillator strength from a Bz $π$ orbital to a virtual orbital of Rydberg character, also involving atomic diffuse functions and partially expanded on the water cluster, were found for the $Geo_{IED}$ series. The energies of such transitions were determined to be more than 2\,eV below the ionization potential of Bz. When the cluster's size increases, similar transitions were found for all structures, the virtual orbitals becoming mainly developed on the H atoms of the water molecules at the edge of the cluster. Given their nature and energy, such transitions could play a role in the photochemistry of aromatic species in interaction with water clusters or ice, such processes being of astrophysical interest.