title: TDDFT diagnostic testing and functional assessment for triazene chromophores.
abstract: A simple 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.
authors.affiliation: Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
authors.forename: Michael J G
authors.initials: MJ
authors.lastname: Peach
authors.forename: C Ruth
authors.initials: CR
authors.lastname: Le Sueur
authors.forename: Kenneth
authors.initials: K
authors.lastname: Ruud
authors.forename: Maxime
authors.initials: M
authors.lastname: Guillaume
authors.forename: David J
authors.initials: DJ
authors.lastname: Tozer
mesh_terms: D005740:Gases / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009570:Nitriles / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013997:Time Factors; D014226:Triazenes / Q000737:chemistry*
chemical_list: C115085:4-(n,n-dimethylamino)benzonitrile; D005740:Gases; D009570:Nitriles; D010455:Peptides; D012997:Solvents; D014226:Triazenes