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Issue 22, 2009
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TDDFT diagnostic testing and functional assessment for triazene chromophores

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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.

Graphical abstract: TDDFT diagnostic testing and functional assessment for triazene chromophores

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Publication details

The article was received on 19 Dec 2008, accepted on 06 Feb 2009 and first published on 03 Mar 2009


Article type: Paper
DOI: 10.1039/B822941D
Citation: Phys. Chem. Chem. Phys., 2009,11, 4465-4470
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    TDDFT diagnostic testing and functional assessment for triazene chromophores

    M. J. G. Peach, C. R. L. Sueur, K. Ruud, M. Guillaume and D. J. Tozer, Phys. Chem. Chem. Phys., 2009, 11, 4465
    DOI: 10.1039/B822941D

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