Classification and Analysis of Molecular Excited States

Kimber, Patrick; Plasser, Felix

doi:10.1016/b978-0-12-821978-2.00053-2

Cited by 9 publications

(28 citation statements)

References 115 publications

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“…Analogous plots for the B 2 u states are presented in Figure S14. First, we observe that the transition densities of all the “+” states are concentrated on the atoms, whereas they are between them for the “–” states. , This is in agreement with the LOC descriptors: The “+” states show enhanced LOC and hence electron and hole are on the same basis function on a given atom, producing transition density on that atom. For the “–” states, on the other hand, the transition density is only formed via “differential overlap” terms, i.e., as the overlap of basis functions on different atoms (see also ref ).…”

Section: Resultssupporting

confidence: 84%

“…Open-shell singlet excited states are generally constructed from at least two Slater determinants. Hence, the derivation of the energy terms is slightly more involved; 25,37 one obtains the expression…”

Section: Methodsmentioning

confidence: 99%

“…Open-shell singlet excited states are generally constructed from at least two Slater determinants. Hence, the derivation of the energy terms is slightly more involved; , one obtains the expression normalΔ E S = ϵ a − ϵ i − ( i i | a a ) + 2 ( i a | i a ) containing the K 2 term and an additional term 2( ia | ia ) that we generally denote as J 2 . The J 2 term only applies to singlet states.…”

Section: Methodsmentioning

confidence: 99%

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Energy Component Analysis for Electronically Excited States of Molecules: Why the Lowest Excited State Is Not Always the HOMO/LUMO Transition

Kimber

Plasser

2023

J. Chem. Theory Comput.

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The ability to tune excited-state energies is crucial to many areas of molecular design. In many cases, this is done based on the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, this viewpoint is incomplete neglecting the many-body nature of the underlying excitedstate wave functions. Within this work, we highlight the importance of two crucial terms, other than orbital energies, that contribute to the excitation energies and show how to quantify them from quantum chemistry computations: a Coulomb attraction and a repulsive exchange interaction. Using this framework, we explain under which circumstances the lowest excited state of a molecule, of either singlet or triplet multiplicity, is not accessed via the HOMO/LUMO transition and show two paradigmatic examples. In the case of the push−pull molecule ACRFLCN, we highlight how the lowest triplet excited state is a locally excited state lying below the HOMO/LUMO charge transfer state due to enhanced Coulomb binding. In the case of the naphthalene molecule, we highlight how the HOMO/LUMO transition (the 1 L a state) becomes the second excited singlet state due to its enhanced exchange repulsion term. More generally, we explain why excitation energies do not always behave like orbital energy gaps, providing insight into photophysical processes as well as methodogical challenges in describing them.

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Section: Resultssupporting

confidence: 84%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Energy Component Analysis for Electronically Excited States of Molecules: Why the Lowest Excited State Is Not Always the HOMO/LUMO Transition

Kimber

Plasser

2023

J. Chem. Theory Comput.

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“…The signicance of these differences is discussed in the literature, for example, in the context of excitons and the L a /L b states in aromatic molecules. 43,[45][46][47][48] Within the doubly excited states, we distinguish between the closed-shell (D CS ) and open-shell (D OS ) limiting cases. In the rst case, exemplied by a pure HOMO 2 / LUMO 2 transition, two electrons are promoted to the same virtual orbital, and the excited state obtains a closed-shell character (D CS ) similar to the The idealized D CS state is single-congurational and behaves like a closed-shell ground state.…”

Section: Classication Of Singly and Doubly Excited Statesmentioning

confidence: 99%

“…The significance of these differences is discussed in the literature, for example, in the context of excitons and the L a /L b states in aromatic molecules. 43,45–48…”

Section: Theorymentioning

confidence: 99%

Classification of doubly excited molecular electronic states

et al. 2023

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Fractional-Electron and Transition-Potential Methods for Core-to-Valence Excitation Energies Using Density Functional Theory

Jana

Herbert

2023

J. Chem. Theory Comput.

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Methods for computing X-ray absorption spectra based on a constrained core hole (possibly containing a fractional electron) are examined. These methods are based on Slater's transition concept and its generalizations, wherein core-to-valence excitation energies are determined using Kohn−Sham orbital energies. Methods examined here avoid promoting electrons beyond the lowest unoccupied molecular orbital, facilitating robust convergence. Variants of these ideas are systematically tested, revealing a best-case accuracy of 0.3−0.4 eV (with respect to experiment) for K-edge transition energies. Absolute errors are much larger for higher-lying near-edge transitions but can be reduced below 1 eV by introducing an empirical shift based on a charge-neutral transition-potential method, in conjunction with functionals such as SCAN, SCAN0, or B3LYP. This procedure affords an entire excitation spectrum from a single fractional-electron calculation, at the cost of ground-state density functional theory and without the need for state-by-state calculations. This shifted transition-potential approach may be especially useful for simulating transient spectroscopies or in complex systems where excitedstate Kohn−Sham calculations are challenging. = c c(1) are estimated as differences between final-state (virtual) energy levels ε υ and the initial-state (core) level ε c . In some cases, a fractional electron may be placed into the lowest unoccupied molecular orbital (LUMO), as described in Section 2, but not into any higher-lying virtual MO. Transition intensities are computed according to

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Classification and Analysis of Molecular Excited States

Cited by 9 publications

References 115 publications

Energy Component Analysis for Electronically Excited States of Molecules: Why the Lowest Excited State Is Not Always the HOMO/LUMO Transition

Energy Component Analysis for Electronically Excited States of Molecules: Why the Lowest Excited State Is Not Always the HOMO/LUMO Transition

Classification of doubly excited molecular electronic states

Fractional-Electron and Transition-Potential Methods for Core-to-Valence Excitation Energies Using Density Functional Theory

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