Density functional approximations for charge transfer excitations with intermediate spatial overlap

Density functional theory is now the method of choice for calculating the electronic structure of complex systems, and time-dependent density functional theory (TDDFT) is now the preferred method for calculating spectroscopic properties of large molecules. The validity of the theory depends mainly on the quality of the approximation to the unknown exchange-correlation energy. In the present paper we consider TDDFT calculations of electronic excitation energies and oscillator strengths. We show that the M06-2X and M08-HX density functionals perform as well as and better than the range-separated CAM-B3LYP functional for charge transfer excitations with intermediate spatial overlap but have better performance for bond energies, noncovalent interactions, and chemical reaction barrier heights for representative systems; we conclude that M06-2X and M08-HX should be preferred for studies requiring the exploration of potential energy surfaces as well as electronic excitation energies, provided that those excitations with the longest-range charge transfer are excluded.