Charge localization and Jahn-Teller distortions in the benzene dimer cation
Jahn-Teller (JT) distortions and charge localization in the benzene dimer cation are analyzed using the equation-of-motion coupled-cluster with single and double substitutions for ionization potentials (EOM-IP-CCSD) method. Ionization of the dimer changes the bonding from non-covalent to covalent, and induces significant geometrical distortions, e.g., shorter interfragment distance and JT displacements. Relaxation along interfragment coordinates lowers the energy of the t-shaped and displaced sandwich isomers by 0.07 eV and 0.23 eV, respectively, whereas JT displacements result in additional 0.18 and 0.23 eV. Energetically, the effect of JT distortion in the dimer is similar to the monomer where JT relaxation lowers energy by 0.18 eV. While the change of the interfragment distance has dramatic spectroscopic consequences, the JT distortion causes only a small perturbation in the electronic spectra. The two geometrical relaxations in the t-shaped isomer lead to opposing effects on hole localization. Intermolecular relaxation leads to increased delocalization, whereas JT ring distortion localizes the charge. In the sandwich isomers, breaking the symmetry by ring rotation does not induce considerable charge localization. The optimization and property calculations were performed using a new implementation of EOM-IP-CCSD energies and gradients in the Q-CHEM electronic structure package.