The Triplet–Singlet Gap in the <i>m</i>-Xylylene Radical: A Not So Simple One

Reta, Daniel; Pal, Arun K.; Moreira, Ibério de P. R.; Datta, Sambhu N.; Illas, Francesc

doi:10.1021/ct400883m

Cited by 60 publications

(47 citation statements)

References 87 publications

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“…Calculation of the triplet–singlet energy gap by quantum chemical methods appears to be notoriously difficult. The vertical 3 B 1 – 1 A 1 energy gap, computed at the ab initio MRCI level by Mañeru et al, depends strongly on the underlying MO basis and the active space, ranging from roughly 1.06 eV when 3 B 1 ‐CASSCF (8,8) MOs are employed to about 0.24 eV for 1 A 1 ‐CASSCF (8,8) MOs. Within the DFT/MRCI approach, a closed‐shell determinant with

\dots 3 b_{2}^{2} 2 a_{2}^{0}

\dots 3 b_{2}^{0} 2 a_{2}^{2}

occupation can be chosen as anchor configuration.…”

Section: Performance Of the Dft/mrci Approachesmentioning

confidence: 99%

The DFT/MRCI method

Marian

Heil

Kleinschmidt

2018

WIREs Comput Mol Sci

148

154

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In the past two decades, the combined density functional theory and multireference configuration interaction (DFT/MRCI) method has developed from a powerful approach for computing spectral properties of singlet and triplet excited states of large molecules into a more general multireference method applicable to states of all spin multiplicities. In its original formulation, it shows great efficiency in the evaluation of singlet and triplet excited states which mainly originate from local one-electron transitions. Moreover, DFT/MRCI is one of the few methods applicable to large systems that yields the correct ordering of states in extended π-systems where double excitations play a significant role. A recently redesigned DFT/MRCI Hamiltonian extends the application range of the method to bichromophores such as hydrogen-bonded or π-stacked dimers and loosely coupled donor-acceptor systems. In conjunction with a restricted-open shell Kohn-Sham optimization of the molecular orbitals, even electronically excited doublet and quartet states can be addressed. After a short outline of the general ideas behind this semi-empirical method and a brief review of alternative approaches combining density functional and multireference wavefunction theory, formulae for the DFT/MRCI Hamiltonian matrix elements are presented and the adjustments of the two-electron contributions are discussed. The performance of the DFT/MRCI variants on excitation energies of organic molecules and transition metal compounds against experimental or ab initio reference data is analyzed and case studies are presented which show the strengths and limitations of the method. Finally, an overview over the properties available from DFT/MRCI wavefunctions and further developments is given.ABBREVIATIONS: ACRXTN, 3-(9,9-dimethylacridin-10[9H]-yl)-9H-xanthen-9-one; AO, atomic orbital; CASSCF, Complete active space self-consistent field; CASPT2, Complete active space second-order perturbation theory; CCSD(T), Coupled-cluster with singles and doubles and perturbative treatment of triples; CC2, Coupled-cluster with approximate treatment of doubles; CD, Circular dichroism; CI, Configuration interaction; CIPSI, configuration interaction by perturbation with multiconfigurational zeroth-order wave functions selected by iterative

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\dots 3 b_{2}^{2} 2 a_{2}^{0}

\dots 3 b_{2}^{0} 2 a_{2}^{2}

occupation can be chosen as anchor configuration.…”

Section: Performance Of the Dft/mrci Approachesmentioning

confidence: 99%

The DFT/MRCI method

Marian

Heil

Kleinschmidt

2018

WIREs Comput Mol Sci

148

154

View full text Add to dashboard Cite

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“…64 Nevertheless, this functional fails to predict the difference in the magnitude of the magnetic coupling constant in a series of ferromagnetic compounds 65 and also performs quite badly in organic diradicals with high spin ground state. 66 The exceedingly large dependence of the calculated results with respect to the choice of the exchange−correlation functional and their shortcoming in describing the ferromagnetic description represents a clear limitation of the DFT-based methods. The general trends and magnetostructural correlations are well described, but the magnitude of the magnetic coupling constants is not.…”

Section: Numerical Validation and Discussionmentioning

confidence: 99%

Handling Magnetic Coupling in Trinuclear Cu(II) Complexes

Reta

Costa

Márquez

et al. 2015

J. Chem. Theory Comput.

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The problem of deriving three different two-body magnetic couplings in three electrons/three centers in a general geometric arrangement is investigated using the trinuclear Cu(II) HAKKEJ complex as a real case example. In these systems, one quartet and two doublet low lying electronic states exist, which define the magnetic spectra. However, the two possible linearly independent energy differences do not provide enough information to extract the three magnetic coupling constants. Here, we show how to obtain these parameters without making any assumption on the symmetry of the system from a combination of density functional- and wave function-based calculations. The density functional calculations explore various broken symmetry solutions and relate the corresponding energy to the expectation value of the Heisenberg Hamiltonian. This allows one to obtain all magnetic couplings, although their magnitude strongly depends on the exchange-correlation functional. Interestingly, a constant ratio between the magnetic coupling constants along a series of investigated functionals is found. This provides an additional equation to be used when relying on energy differences between spin states, which in turn allow solving the Heisenberg spectrum. The magnetic couplings thus obtained are compared to the experiment. Implications for the appropriate interpretation of the experiment and for the study of more complex systems are discussed.

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“…However, it fails to describe the singlet-triplet gap of mxylylene, a prototypal organic diradical. 66 Yet, it is likely that the poor performance of the LC-ωPBE functional comes from the value of the range separation parameter which is chosen to reproduce thermochemistry related properties of molecules with closed shell structure. These properties are dominated by valence electrons and the bad performance in describing core level BE's could perhaps have been anticipated.…”

Section: A Overall Accuracy Of Calculated N(1s) Be'smentioning

confidence: 99%

Prediction of core level binding energies in density functional theory: Rigorous definition of initial and final state contributions and implications on the physical meaning of Kohn-Sham energies

Bellafont

Bagus

Illas

2015

The Journal of Chemical Physics

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127

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A systematic study of the N(1s) core level binding energies (BE's) in a broad series of molecules is presented employing Hartree-Fock (HF) and the B3LYP, PBE0, and LC-BPBE density functional theory (DFT) based methods with a near HF basis set. The results show that all these methods give reasonably accurate BE's with B3LYP being slightly better than HF but with both PBE0 and LCBPBE being poorer than HF. A rigorous and general decomposition of core level binding energy values into initial and final state contributions to the BE's is proposed that can be used within either HF or DFT methods. The results show that Koopmans' theorem does not hold for the Kohn-Sham eigenvalues. Consequently, Kohn-Sham orbital energies of core orbitals do not provide estimates of the initial state contribution to core level BE's; hence, they cannot be used to decompose initial and final state contributions to BE's. However, when the initial state contribution to DFT BE's is properly defined, the decompositions of initial and final state contributions given by DFT, with several different functionals, are very similar to those obtained with HF. Furthermore, it is shown that the differences of Kohn-Sham orbital energies taken with respect to a common reference do follow the trend of the properly calculated initial state contributions. These conclusions are especially important for condensed phase systems where our results validate the use of band structure calculations to determine initial state contributions to BE shifts.

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The Triplet–Singlet Gap in the m-Xylylene Radical: A Not So Simple One

Cited by 60 publications

References 87 publications

The DFT/MRCI method

The DFT/MRCI method

Handling Magnetic Coupling in Trinuclear Cu(II) Complexes

Prediction of core level binding energies in density functional theory: Rigorous definition of initial and final state contributions and implications on the physical meaning of Kohn-Sham energies

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