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Outer-valence Electron Spectra of Prototypical Aromatic Heterocycles from an Optimally Tuned Range-Separated Hybrid Functional.

Egger DA, Weissman S, Refaely-Abramson S, Sharifzadeh S, Dauth M, Baer R, Kümmel S, Neaton JB, Zojer E, Kronik L - J Chem Theory Comput (2014)

Bottom Line: In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states.We discuss in detail the sole exception found-a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold.Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

View Article: PubMed Central - PubMed

Affiliation: Institute of Solid State Physics, Graz University of Technology , 8010 Graz, Austria ; Department of Materials and Interfaces, Weizmann Institute of Science , Rehovoth 76100, Israel.

ABSTRACT
Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. Phys. Rev. Lett. 2012, 109, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1-0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception found-a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

No MeSH data available.


Charge-density difference between neutral andcation (left) andLUMO of cation (right), obtained using two different doublet configurations(a and b) of the pyridine cation. The configuration denoted by b isthe energetically more stable one. In the charge-density differenceplots, red (blue) regions denote areas of electron density depletion(accumulation) as a consequence of the ionization process. (c) HOMOof the neutral pyridine molecule.
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fig2: Charge-density difference between neutral andcation (left) andLUMO of cation (right), obtained using two different doublet configurations(a and b) of the pyridine cation. The configuration denoted by b isthe energetically more stable one. In the charge-density differenceplots, red (blue) regions denote areas of electron density depletion(accumulation) as a consequence of the ionization process. (c) HOMOof the neutral pyridine molecule.

Mentions: We illustrate the possible complications associated with differention configurations by considering pyridine—one of the moleculesof Figure 1, which is discussed extensivelyin the results section. When tuning γ using the above-describedapproach, it is important to ensure that the character of the HOMO(related to the left-hand term in eq 3) correspondsto the “hole density,” defined as the charge-densitydifference of the neutral and charged states (and, consequently, relatedto the right-hand term in eq 3). For pyridine,the self-consistent solution of the GKS equation with the RSH functionalwas found to lead to two different doublet configurations of the cation,depending on the initial guess used in the procedure. These two configurationscorrespond to two qualitatively different hole densities (see Figure 2, left part). As shown in the right part of Figure 2, the reason for the different hole densities isthat the two cation configurations possess two different LUMO orbitals;i.e., the two cationic ground states represent two different ionizationprocesses. The main difference is that the electron “loss”is, in one case, from a π orbital and, in the other case, froma σ orbital. These two cationic configurations are energeticallyclose, which is consistent with the observation that the HOMO andthe HOMO–1 of pyridine are very close in energy (videinfra). Importantly, however, only the “hole density”depicted in Figure 2b—which is associatedwith the configuration lower in energy, i.e., the true ground statepredicted for the cation—corresponds to the HOMO of pyridine(see Figure 2c). Therefore, one has to ensurethat the cationic state shown in Figure 2bis indeed the one entering the tuning procedure, in order to retainconsistency for the orbital energies and total energies required ineq 3.


Outer-valence Electron Spectra of Prototypical Aromatic Heterocycles from an Optimally Tuned Range-Separated Hybrid Functional.

Egger DA, Weissman S, Refaely-Abramson S, Sharifzadeh S, Dauth M, Baer R, Kümmel S, Neaton JB, Zojer E, Kronik L - J Chem Theory Comput (2014)

Charge-density difference between neutral andcation (left) andLUMO of cation (right), obtained using two different doublet configurations(a and b) of the pyridine cation. The configuration denoted by b isthe energetically more stable one. In the charge-density differenceplots, red (blue) regions denote areas of electron density depletion(accumulation) as a consequence of the ionization process. (c) HOMOof the neutral pyridine molecule.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4020925&req=5

fig2: Charge-density difference between neutral andcation (left) andLUMO of cation (right), obtained using two different doublet configurations(a and b) of the pyridine cation. The configuration denoted by b isthe energetically more stable one. In the charge-density differenceplots, red (blue) regions denote areas of electron density depletion(accumulation) as a consequence of the ionization process. (c) HOMOof the neutral pyridine molecule.
Mentions: We illustrate the possible complications associated with differention configurations by considering pyridine—one of the moleculesof Figure 1, which is discussed extensivelyin the results section. When tuning γ using the above-describedapproach, it is important to ensure that the character of the HOMO(related to the left-hand term in eq 3) correspondsto the “hole density,” defined as the charge-densitydifference of the neutral and charged states (and, consequently, relatedto the right-hand term in eq 3). For pyridine,the self-consistent solution of the GKS equation with the RSH functionalwas found to lead to two different doublet configurations of the cation,depending on the initial guess used in the procedure. These two configurationscorrespond to two qualitatively different hole densities (see Figure 2, left part). As shown in the right part of Figure 2, the reason for the different hole densities isthat the two cation configurations possess two different LUMO orbitals;i.e., the two cationic ground states represent two different ionizationprocesses. The main difference is that the electron “loss”is, in one case, from a π orbital and, in the other case, froma σ orbital. These two cationic configurations are energeticallyclose, which is consistent with the observation that the HOMO andthe HOMO–1 of pyridine are very close in energy (videinfra). Importantly, however, only the “hole density”depicted in Figure 2b—which is associatedwith the configuration lower in energy, i.e., the true ground statepredicted for the cation—corresponds to the HOMO of pyridine(see Figure 2c). Therefore, one has to ensurethat the cationic state shown in Figure 2bis indeed the one entering the tuning procedure, in order to retainconsistency for the orbital energies and total energies required ineq 3.

Bottom Line: In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states.We discuss in detail the sole exception found-a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold.Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

View Article: PubMed Central - PubMed

Affiliation: Institute of Solid State Physics, Graz University of Technology , 8010 Graz, Austria ; Department of Materials and Interfaces, Weizmann Institute of Science , Rehovoth 76100, Israel.

ABSTRACT
Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. Phys. Rev. Lett. 2012, 109, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1-0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception found-a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

No MeSH data available.