Coupled-cluster calculations of the excitation energies of benzene and the azabenzenes

A series of equation-of-motion coupled-cluster (EOM-CC) calculations of the vertical excitation energies of benzene, pyridine, pyrazine, pyrimidine, pyridazine, symmetric triazine, and symmetric tetrazine have been performed. Single and double excitations have been included fully, and a noniterative approximation has been used to estimate triple excitation effects [the EOM-CCSD(T̃) method]. The basis set contains polarization functions and has reasonable diffuseness. Comparison is made with experimental data and second-order perturbation theory complete active space (CASPT2) theoretical data. The average EOM-CCSD(T̃) error for π→π* transitions is 0.11 eV and the error for n→π* transitions is 0.15 eV. Based on these small errors, several uncertain assignments for pyrazine and pyrimidine are substantiated.