Charge-resonance excitations in symmetric molecules - Comparison of linear response DFT with CC3 for the excited states of a model dimer

We present a study on the excited states of an ethylene dimer as to investigate the presence of and perturbation from low-lying charge-resonance states calculated by linear response density functional theory (DFT) using the B3LYP and CAM-B3LYP functionals. The calculations are compared to a reference CC3 calculation revealing a better description of the excited states by CAM-B3LYP than that of B3LYP. The Λ parameter introduced by Peach et al. [M.J.G. Peach, P. Benfield, T. Helgaker, D.J. Tozer, J. Chem. Phys. 128 (2008) 044118] does not always reveal the problematic charge-resonance states obtained with B3LYP. The generality of the problem is addressed by taking pentacene as an example.