Ground state and vertical excitation energies of the diazene isomers with the coupled cluster method

Abstract State‐of‐the‐art coupled cluster methods were used to study ground and excited states of the diazene isomers. Three coupled cluster variants were applied in the ground state calculations: CCSD, CCSDT, and CCSDT(Qf) for basis sets up to aug‐cc‐pV5Z. The ground state equilibrium geometry was found using combination of the analytical and numerical gradients. The computed values compare very well with experimental data if the latter are available. The vertical excitation energies were obtained with the EOM‐CCSD and EOM‐CCSDT approaches for the same set of basis sets. The importance of the connected triple excitations in the accurate evaluation of the excitation energy is discussed. Out of the four lowest lying excited states studied for each isomer at least one belongs to the category for which an inclusion of triple excitations is crucial for its adequate description. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008