Adiabatic Excitation Energies Research Articles

Molecular properties of excited electronic state: formalism, implementation, and applications of analytical second energy derivatives within the framework of the time-dependent density functional theory/molecular mechanics.

This work extends our previous works [J. Liu and W. Z. Liang, J. Chem. Phys. 135, 014113 (2011); J. Liu and W. Z. Liang, J. Chem. Phys. 135, 184111 (2011)] on analytical excited-state energy Hessian within the framework of time-dependent density functional theory (TDDFT) to couple with molecular mechanics (MM). The formalism, implementation, and applications of analytical first and second energy derivatives of TDDFT/MM excited state with respect to the nuclear and electric perturbations are presented. Their performances are demonstrated by the calculations of adiabatic excitation energies, and excited-state geometries, harmonic vibrational frequencies, and infrared intensities for a number of benchmark systems. The consistent results with the full quantum mechanical method and other hybrid theoretical methods indicate the reliability of the current numerical implementation of developed algorithms. The computational accuracy and efficiency of the current analytical approach are also checked and the computational efficient strategies are suggested to speed up the calculations of complex systems with many MM degrees of freedom. Finally, we apply the current analytical approach in TDDFT/MM to a realistic system, a red fluorescent protein chromophore together with part of its nearby protein matrix. The calculated results indicate that the rearrangement of the hydrogen bond interactions between the chromophore and the protein matrix is responsible for the large Stokes shift.

Conductor‐like screening model for relaxed excited states: Implementation in the semiempirical method MSINDO

Two approaches to treat solvent polarization and reorientation effects for excited states of molecules and surfaces have been implemented in the recently developed MSINDO-sCIS method (Gadaczek, Krause, Hintze, Bredow, J. Chem. Theory Comput. 2011, 7, 3675). They allow for an efficient calculation of analytical energy gradients and hence open the opportunity to investigate fluorescence effects or photochemical reactions in solution for large molecules that are difficult to treat with high-level methods. Both approaches are based on the conductor-like screening model (COSMO) (Klamt and Schüürmann, J. Chem. Soc., Perkin Trans. 1993, 2, 799) in combination with the configuration interaction singles (CIS) method (Foresman, Head-Gordon, Pople, and Frisch, J. Phys. Chem. 1992, 96, 135). The paper gives a brief outline of the theoretical background. As a first application, solvent shifts of three well-studied, environment-sensitive fluorescent dyes (Kucherak, Didier, Mély, and Klymchenko, J. Phys. Chem. Lett. 2010, 1, 616) have been calculated and compared with experimental results and standard time-dependent density functional theory. A statistical evaluation of MSINDO-COSMO-sCIS is provided for a set of 39 molecules suggested recently by Jacquemin et al. (Jacquemin, Planchat, Adamo, and Mennucci, J. Chem. Theory Comput. 2012, 8, 2359). Calculated vertical and adiabatic excitation energies and fluorescence energies are compared to experimental data.

Molecular structure and spectra of titanium and vanadium trifluorides: Complete basis set CCSD(T) study

Quantum chemical study on TiF3 and VF3 molecules was carried out using the CCSD(T) coupled cluster method using the triple-, quadruple-, and quintuple-zeta basis set and an extrapolation to the complete basis set limit. The methods of multireference configuration interaction MRCISD+Q and perturbation theory MCQDPT2 were used also. The symmetry of the ground electronic state was determined: 2 A′1 and 3 E″ in TiF3 and VF3, respectively. The adiabatic excitation energies were evaluated: AEE(TiF3, \(\tilde A^2 E'' \leftarrow \tilde X^2 A'_1 \)) = 5000 cm−1, AEE(VF3, \(\tilde A^3 A'_2 \leftarrow \tilde X^3 E''\)) = 1000 cm−1. The Jahn-Teller effect in \(\tilde A^2 E''\) state of TiF3 and \(\tilde X^3 E''\) state of VF3 was investigated. The computed Jahn-Teller stabilization energy D 3h → C 2v amounts to 555 and 292 cm−1, respectively. The spin-orbit coupling effect on the VF3 molecular structure and spectrum of electronic states is shown to be quite significant. The calculated vibrational frequencies of TiF3 are in excellent agreement with IR spectroscopy data. The atomization enthalpies were evaluated: Δat H 298 pO = 430 kcal/mol (TiF3), 393 kcal/mol (VF3).

Electronic states of the [formula omitted] ion studied using multiconfiguration second-order perturbation theory

Electronic states of the m-C6H4Cl2+ ion have been studied based on the multiconfiguration second-order perturbation theory (CASPT2) geometry optimization calculations. For the six lowest-lying states, geometries and adiabatic excitation energies (T0) were calculated. Based on the CASPT2 T0 calculations, the X∼, A∼, B∼, C∼, D∼, and E∼ states of m-C6H4Cl2+ were assigned to 12A2, 12B1, 12B2, 12A1, 22B1, and 22A2, respectively. The current calculations resolved the differences of experimental assignment regarding the B∼ state and the assignments on the C∼ and D∼ states were different from the previous published works.

Electronic Transitions of C6H4+ Isomers: Neon Matrix and Theoretical Studies

Three open-chain isomers of C6H4(+) and two cyclic ones were detected following mass-selective trapping in 6 K neon matrixes. The open-chain cations 5-hexene-1,3-diyne (CH2═CH-CC-CC-H)(+) and cis- (cis-HCC-CH═CH-CCH)(+) and trans-3-hexene-1,5-diyne (trans-HCC-CH═CH-CCH)(+), possess two absorption systems commencing at 609 and 373, 622 and 385, and 585 and 373 nm, respectively. They are assigned to the 1 (2)A" and 2 (2)A" ← X (2)A", 1(2)A2 and 2 (2)A2 ← X (2)B1, and 1 (2)Bg and 2 (2)B(g) ← X (2)A(u) electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the 1 (2)A" ← X (2)A" electronic transitions of propargyl cyclopropene and 2 (2)B1 ← X (2)A2 of o-benzyne cation structures. The assignment of the electronic transitions is based on theoretical vertical excitation energies calculated with CASPT2 and EOMEE-CCSDT methods for 12 isomers of C6H4(+). These have been carried out at the geometries optimized using several ab initio methods. Adiabatic excitation energies were calculated for the five identified isomers of C6H4(+).

Electronic structure calculations for the study of D-π-A organic sensitizers: Exploring polythiophene linkers

In this work we present a detailed study of the atomic and electronic structure of a collection of push–pull organic dyes for high-performance sensitized solar cells (DSSCs). We compare the computed photophysical properties of donor-bridge-acceptor (D-π-A) dyes with polythiophene (Tn) or polycyclopentadithiophene (Cn) conjugated linkers with up to four fused thiophene rings. Excitation energies to lowest excited singlet state have been rationalized by means of fragment and molecular orbitals. Vertical and adiabatic excitation energies are systematically lower for the Cn family and become smaller with the length of the molecular conjugation. We discuss a large variety of computational techniques for the characterization of the charge transfer (CT) nature of the electronic excitation. In addition to standard procedures to quantify CT character, we propose and explain several novel interaction based measures of CT. Finally, we have computed ground and excited state oxidation potentials (GSOP and ESOP) with long-range corrected (LRC) functional.

Electronic and magnetic properties of 3d transition metal-doped strontium clusters: Prospective magnetic superatoms

Structural, electronic and magnetic properties of 3d transition metal doped strontium clusters are studied using first-principles electronic structure methods based on density functional theory. Clusters with enhanced kinetic and thermodynamic stability are identified by studying their hardness, second order energy difference and adiabatic spin excitation energy. CrSr9 and MnSr10 are found to have enhanced stability. They retain their structural identities in assemblies, and are classified as magnetic superatoms. A qualitative understanding of the magnetic coupling between two cluster units is arrived at. Reactivity of these superatoms with O2 molecule is also studied. Prospects for using these magnetic superatoms in applications are discussed.

THE STUDY OF THE PROPERTIES OF THE GROUND- AND LOW-LYING EXCITED STATES OF ASH2, ${\rm ASH}_{2}^{+}$ AND ${\rm ASH}_{2}^{-}$

The properties of the ground and excited states of AsH2 , [Formula: see text] and [Formula: see text] have been investigated by using symmetry-adapted-cluster (SAC)/symmetry-adapted-cluster configuration interaction (SAC-CI) method. The geometry of the ground state of AsH2 is optimized at SAC method with different basis sets. The calculated results with cc-pVTZ and cc-pVQZ basis sets are in very good agreement with the experimental and previous theoretical data. The geometry and the properties of eight low-lying electronic excited states of AsH2 are obtained at SAC-CI/cc-pVTZ and SAC-CI/cc-pVQZ level, including geometries, vertical excitation energies, adiabatic excitation energies, transition dipole moments, and oscillation strengths. Employing the same theoretical level as AsH2 , the geometries, adiabatic ionization potentials (AIPs), and vertical ionization potentials (VIPs) of the ground and eight low-lying electronic states of [Formula: see text] are investigated as well as the geometries, vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) of nine electronic states of [Formula: see text]. Comparing with the available experimental or previous theoretical data, the SAC/SAC-CI/cc-pVTZ and SAC/SAC-CI/cc-pVQZ results are reliable for AsH2 , [Formula: see text] and [Formula: see text]. The predicted results can afford the useful information for one to deeply investigate them from the spectral experiment.

Calculating excited state properties using Kohn-Sham density functional theory

The accuracy of excited states calculated with Kohn-Sham density functional theory using the maximum overlap method has been assessed for the calculation of adiabatic excitation energies, excited state structures, and excited state harmonic and anharmonic vibrational frequencies for open-shell singlet excited states. The computed Kohn-Sham adiabatic excitation energies are improved significantly by post self-consistent field spin-purification, but remain too low compared with experiment with a larger error than time-dependent density functional theory. Excited state structures and vibrational frequencies are also improved by spin-purification. The structures show a comparable accuracy to time-dependent density functional theory, while the harmonic vibrational frequencies are found to be more accurate for the majority of vibrational modes. The computed harmonic vibrational frequencies are also further improved by perturbative anharmonic corrections, suggesting a good description of the potential energy surface. Overall, excited state Kohn-Sham density functional theory is shown to provide an efficient method for the calculation of excited state structures and vibrational frequencies in open-shell singlet systems and provides a promising technique that can be applied to study large systems.

Geometric Structures, Excitation Energies and Dipole Moments of the Ground and Excited States of TiO<sub>2</sub>

The geometries of the ground and excited states of titanium dioxide,1A1,1B2, 3B2,1B1, 3B1,1A2, and 3A2 have been optimized using M ller-Plesset second-order perturbation theory, density functional theory B3 LYP, and time-dependent density functional theory TD-B3LYP methods.1A1,1B2, 3B2,1B1, 3B1, and 3B1 have bent structures, while 1A2 and 3A2 have symmetrical linear structures. The bond angles of1B2, 3 B2,1B1, and 3B1 correlate directly with the magnitudes of the corresponding bond dipole moments. Vertical and adiabatic excitation energies have been computed with complete active space self-consistent field (CASSCF) CASSCF(6,6), CASSCF(8,8), multi-reference configuration interaction (MRCI), and TD-B3 LYP. For1B2,3B2, and 1B1, the excitation energies calculated with MRCI/CASSCF(6,6) are much closer to the experimental values than the results calculated using other methods. For excited states 3B1,1A2, and 3A2, excitation energies calculated with CASSCF(6,6), CASSCF(8,8), MRCI, and TD-B3 LYP are almost consistent with theoretical results available in the literature. Dipole moments of the ground and excited states have been computed with B3 LYP and TD-B3 LYP. The calculated dipole moments of 1A1 and 1B2 agree well with experimental data. The atomic charges of TiO2 in ground and excited states have been calculated with the atomic dipole moment corrected Hirshfeld population method. This calculation revealed that changes of dipole moments from the ground state to the excited states are related to electron transfer from the oxygen atom to the titanium atom. During the above calculations, the influences of the basis sets cc-pVDZ, cc-pVTZ, and cc-pVQZ were also investigated.

The electronic structure of pyracene: a spectroscopic and computational study

We report a synthetic, spectroscopic and computational study of the polycyclic aromatic molecule pyracene, which contains aliphatic five-membered rings annealed to a naphthalene chromophore. An improved route to synthesize the compound is described. Gas-phase IR and solid-state Raman spectra agree with a ground-state D2h structure. The electronically excited S1 A(1)B3u state has been studied by resonance-enhanced multiphoton ionisation. An adiabatic excitation energy T0 = 30,798 cm(-1) (3.818 eV) was determined. SCS-ADC(2) calculations found a D2h minimum energy structure of the S1 state and yielded an excitation energy of +3.98 eV, including correction for zero point vibrational energy. The spectrum shows a rich low-frequency vibrational structure that can be assigned to the overtones of out-of-plane deformation modes of the five-membered rings by comparison with computations. The appearance of these modes as well as the frequency reduction in the excited state indicate that the potential in the S1 state is very flat. At higher excess energies most bands can be assigned to fundamentals, overtones and combination bands of either totally symmetric ag modes or of b2g modes that appear due to vibronic coupling. Lifetimes between 43 ns and 76 ns were measured for a number of vibronic bands. For the S2 state an equilibrium geometry with a non-planar carbon framework was computed. In addition a signal from the pyracene dimer was present. The spectrum shows several broad and structureless transitions. The origin band has a maximum at around 329 nm (30,400 cm(-1)). Again lifetimes between 60 ns and 70 ns were found. The dimer ion signal rises within less than 10 ps. Computations show that a crossed geometry with the long axis of one unit aligned with the short axis of the second constitutes the most stable structure. The broadening of the bands is most likely caused by excimer formation.

Taming the low-lying electronic states of FeH

The low-lying electronic states (X (4)Δ, A (4)Π, a (6)Δ, b (6)Π) of the iron monohydride radical, which are especially troublesome for electronic structure theory, have been successfully described using a focal point analysis (FPA) approach that conjoined a correlation-consistent family of basis sets up to aug-cc-pwCV5Z-DK with high-order coupled cluster theory through hextuple (CCSDTQPH) excitations. Adiabatic excitation energies (T(0)) and spectroscopic constants (r(e), r(0), B(e), B(0), D(e), ω(e), v(0), α(e), ω(e)x(e)) were extrapolated to the valence complete basis set Douglas-Kroll (DK) aug-cc-pwCV∞Z-DK CCSDT level of theory, and additional treatments accounted for higher-order valence electron correlation, core correlation, spin-orbit coupling, and the diagonal Born-Oppenheimer correction. The purely ab initio FPA approach yields the following T(0) results (in eV) for the lowest spin-orbit components of each electronic state: 0 (X (4)Δ) < 0.132 (A (4)Π) < 0.190 (a (6)Δ) < 0.444 (b (6)Π). The computed anharmonic fundamental vibrational frequencies (v(0)) for the (4,6)Δ electronic states are within 3 cm(-1) of experiment and provide reliable predictions for the (4,6)Π states. With the cc-pVDZ basis set, even CCSDTQPH energies give an incorrect ground state of FeH, highlighting the importance of combining high-order electron correlation treatments with robust basis sets when studying transition-metal radicals. The FPA computations provide D(0) = 1.86 eV (42.9 kcal mol(-1)) for the 0 K dissociation energy of FeH and Δ(f)H(298) (∘) [FeH((g))] = 107.7 kcal mol(-1) for the enthalpy of formation at room temperature. Despite sizable multireference character in the quartet states, high-order single-reference coupled cluster computations improve the spectroscopic parameters over previous multireference theoretical studies; for example, the X (4)Δ → A (4)Π and a (6)Δ → b (6)Π transition energies are reproduced to 0.012 and 0.002 eV, respectively, while the error for the problematic X (4)Δ → a (6)Δ intercombination excitation is reduced from at least 0.17 eV to about 0.04 eV.

Harnessing the meta-generalized gradient approximation for time-dependent density functional theory

Density functionals within the meta-generalized gradient approximation (MGGA) are widely used for ground-state electronic structure calculations. However, the gauge variance of the kinetic energy density τ confounds applications of MGGAs to time-dependent systems, excited states, magnetic properties, and states with strong spin-orbit coupling. Becke and Tao used the paramagnetic current density to construct a gauge invariant generalized kinetic energy density τ. We show that τ(W)≤τ, where τ(W) is the von Weizsäcker kinetic energy density of a one-electron system. Thus, replacing τ by τ leads to current-dependent MGGAs (cMGGAs) that are not only gauge invariant but also restore the accuracy of MGGAs in iso-orbital regions for time-dependent and current-carrying states. The current dependence of cMGGAs produces a vector exchange-correlation (XC) potential in the time-dependent adiabatic Kohn-Sham (KS) equations. While MGGA response properties of current-free ground states become manifestly gauge-variant to second order, linear response properties are affected by a new XC kernel appearing in the cMGGA magnetic orbital rotation Hessian. This kernel reflects the first-order coupling of KS orbitals due to changes in the paramagnetic current density and has apparently been ignored in previous MGGA response implementations. Inclusion of the current dependence increases total computation times by less than 50%. Benchmark applications to 109 adiabatic excitation energies using the Tao-Perdew-Staroverov-Scuseria (TPSS) MGGA and its hybrid version TPSSh show that cMGGA excitation energies are slightly lower than the MGGA ones on average, but exhibit fewer outliers. Similarly, the optical rotations of 13 small organic molecules show a small but systematic improvement upon inclusion of the magnetic XC kernel. We conclude that cMGGAs should replace MGGAs in all applications involving time-dependent or current-carrying states.

Theoretical computation of low-lying electronic states of HCNS: A CASPT2 study

Complete active space self-consistent field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations in conjunction with the aug-cc-pVTZ basis set have been used to investigate the low-lying electronic states of thiofulminic acid (HCNS), HCNS+, and HCNS−. The result of geometry optimization using CASPT2/aug-cc-pVTZ shows that theoretically determined geometric parameters and harmonic vibrational frequencies for the HCNS ground state X1Σ+(X1A′) are in agreement with previous studies. The ionization energies, the electron affinity energies, the adiabatic excitation energies, and vertical excitation energies have been calculated and the corresponding cation and anion states are identified. By calculating adiabatic electron affinity, the states of HCNS− have been identified to contain both π orbital states (X2A′ and 12A″) and dipole-bond states (14A′ and 14A″). © 2012 Wiley Periodicals, Inc.

The ZnO2+ dication in the gas phase

Our high-level ab initio calculations show that the ZnO2+ electronic ground state of 3Σ− symmetry is metastable with a bond length of 2.066Å, an effective dissociation energy of 1.42eV, and a double ionization energy of ZnO of 26.61eV. Spectroscopic constants, adiabatic excitation energies, tunneling and spin–orbit induced predissociation lifetimes for the vibrational levels in the low-lying electronic states of ZnO2+ were determined and the metastability of the dication was discussed. For the lower electronic states of ZnOn+ (n=0, 1, 2) species, the spectroscopic characteristics of the Λ–S states were calculated and the double ionization spectrum of ZnO was simulated.

CASPT2 and CCSD(T) calculations of dipole moments and polarizabilities of acetone in excited states

The acetone molecule is investigated in its ground state and valence 1,3n-π*, 1,3π-π*, and 1,3σ-π* excited states and Rydberg 1,3n-3s, 1,3π-3 , 1,3n-3py and 1,3π-3py states using the CASSCF, CASPT2, and CCSD(T) methods. Equilibrium geometries of excited states are obtained and their changes with respect to the ground state are discussed. For most excited states the C2v symmetry of the ground state is lowered to the Cs symmetry. A series of valence vertical and adiabatic excitation energies is presented along with excitation energies for Rydberg states. The main body of the paper contains Finite-Field Perturbation Theory (FFPT) calculations of electric properties of the vertically as well as geometry relaxed excited states. Dipole moments of valence excited states decrease significantly upon excitation, being about one half of the ground state dipole moment. Polarizabilities usually change upon excitation much less (increase by about 30%) but hyperpolarizabilities are enhanced up to one or two orders of magnitude. The orientation of the dipole moment is reversed in some vertically excited Rydberg states. Properties of the ground and excited states are discussed considering alterations of the electronic structure and shifts in the geometry.

Search for a Small Chromophore with Efficient Singlet Fission: Biradicaloid Heterocycles

Of the five small biradicaloid heterocycles whose S(1), S(2), T(1), and T(2) adiabatic excitation energies were examined by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists.

Spectroscopy of the Simplest Criegee Intermediate CH2OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra

CH(2)OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas-phase studies on CH(2)OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH(2)OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH(2)OO. In order to encourage more direct studies on CH(2)OO and other Criegee intermediates, the electronic and photoelectron spectra of CH(2)OO have been simulated using high level electronic structure calculations and Franck-Condon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH(2)OO were calculated with TDDFT, EOM-CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low-lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH(2) OO corresponds to the B(1)A' ← X(1)A' absorption. It is a broad band in the region 250-450 nm showing extensive structure in vibrational modes involving O-O stretching and C-O-O bending. Evidence is presented to show that the electronic absorption spectrum of CH(2)OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH(2)OO was prepared in this earlier work from the reaction of CH(2)I with O(2) and that the assignment of the observed spectrum solely to CH(2)IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH(2)OO consists of two overlapping vibrationally structured bands corresponding to one-electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.

The vacuum UV photoabsorption spectroscopy of the geminal ethylene difluoride (1,1-C2H2F2). The vibrational structure and its analysis

The vacuum UV photoabsorption spectrum of 1,1-C2H2F2 has been examined in detail between 6eV and 25eV photon energy by using synchrotron radiation. The broad band observed at 7.52eV includes the π→π∗ and the 2b1→3s Rydberg transitions. An analysis is proposed and applied to the fine structure belonging to these transitions. For the π(2b1)→π∗ transition, one long vibrational progression is observed with ω2=1475±80cm−1 combined with one quantum of vibration of ω4=976±24cm−1 and its adiabatic excitation energy is determined at 6.742eV. The 2b1→3s Rydberg transition is characterized by a single progression with ω2=1532±80cm−1 starting at 6.957eV. These vibrations are ascribed to the CC and C–F stretching motions respectively. The abundant structure observed between 8.2eV and 11.2eV has been analyzed in terms of vibronic transitions to ns (δ=0.98) and two different np (δ=0.50 and 0.31) and nd (δ=0.13 and 0.044) Rydberg series all converging to the 1,1-C2H2F2+(X˜2B1) ionic ground state. An analysis of the associated vibrational structure of the individual Rydberg states has been attempted providing average values of the wavenumbers ω2=1549±16cm−1, ω4=839±40cm−1 and ω5=589±16cm−1. Eight other Rydberg states were also analyzed. The vacuum UV spectrum of 1,1-C2H2F2 has been recorded for the first time above 11.2eV and up to 25eV. Several broad and strong bands are tentatively assigned to transitions to Rydberg states which are members of Rydberg series converging to excited ionic states of 1,1-C2H2F2. For several of these Rydberg states vibrational, structures are observed and assignments are proposed.

Charge-Transfer Excitations in Uranyl Tetrachloride ([UO2Cl4]2–): How Reliable are Electronic Spectra from Relativistic Time-Dependent Density Functional Theory?

Four-component relativistic time-dependent density functional theory (TD-DFT) is used to study charge-transfer (CT) excitation energies of the uranyl molecule as well as the uranyl tetrachloride complex. Adiabatic excitation energies and vibrational frequencies of the excited states are calculated for the lower energy range of the spectrum. The results for TD-DFT with the CAM-B3LYP exchange-correlation functional for the [UO(2)Cl(4)](2-) system are in good agreement with the experimentally observed spectrum of this species and agree also rather well with other theoretical data. Use of the global hybrid B3LYP gives qualitatively correct results, while use of the BLYP functional yields results that are qualitatively wrong due to the too low CT states calculated with this functional. The applicability of the overlap diagnostic of Peach et al. (J. Chem. Phys.2008, 128, 044118) to identify such CT excitations is investigated for a wide range of vertical transitions using results obtained with three different approximate exchange-correlation functionals: BLYP, B3LYP, and CAM-B3LYP.