Michael J. G. Peach, C. Ruth Le Sueur, Kenneth Ruud, Maxime Guillaume, David J. Tozer, Phys. Chem. Chem. Phys., 11, 4465–4470, 2009.
This paper includes the first practical application of the Lambda diagnostic, where we apply the diagnostic to the excitations in a theoretically challenging triazene chromophore. The usefulness of the diagnostic is clearly illustrated, as is the excellent performance of the CAM-B3LYP functional, whose accuracy approaches that of wavefunction-based data for this molecule.
Further information, including details of subsequent work in this area, can be found on the TDDFT diagnostic research page. For the abstract, and access to the full text, see below.
Asimple diagnostic test based on orbital overlap [M. J. G. Peach et al., J. Chem. Phys., 2008,128, 044118] may be used to help judge the reliability of excitation energies in time-dependent density functional theory (TDDFT) when using generalized gradient approximation (GGA) and hybrid functionals. Orbital plots are used to illustrate the test for a model tripeptide and for 4-(N,N-dimethylamino)benzonitrile, which are representative of systems containing low- and high-overlap charge-transfer excitations. The scheme is then applied to a series of triazene chromophores in solvent, highlighting the relationship between overlap and oscillator strength and its implications for theoretical absorption spectra. No low-overlap excitations are observed with a hybrid functional; a single one is identified using a GGA. To assess the diagnostic test and to judge functional performance, gas phase triazene TDDFT excitations are compared with correlated ab initio values. The diagnostic test correctly identifies two low-overlap problematic GGA excitations. However, it does not identify another problematic excitation where the electron is excited to a spatially extended orbital, which necessarily has reasonable overlap with the occupied orbital; an improved diagnostic quantity is required for such cases. The best agreement between TDDFT and correlated ab initio excitations is obtained using a Coulomb-attenuated functional; the errors are significantly smaller than from the GGA and hybrid functionals. The study provides further support for the high quality excitations from Coulomb-attenuated functionals, negating the need for diagnostic tests.