Abstract
We investigate the role of Kekulé and non-Kekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using thermally-assisted-occupation density functional theory (TAO-DFT), an efficient electronic structure method for the study of large ground-state systems with strong static correlation effects. Our results reveal that the studies of Kekulé and non-Kekulé structures qualitatively describe the radical character of alternant PAHs, which could be useful when electronic structure calculations are infeasible due to the expensive computational cost. In addition, our results support previous findings on the increase in radical character with increasing system size. For alternant PAHs with the same number of aromatic rings, the geometrical arrangements of aromatic rings are responsible for their radical character.
Highlights
We investigate the role of Kekulé and non-Kekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using thermally-assisted-occupation density functional theory (TAO-DFT), an efficient electronic structure method for the study of large ground-state systems with strong static correlation effects
For the alternant PAHs with the same number of aromatic rings, the geometrical arrangements of aromatic rings are shown to be responsible for the radical character, and it is observed that the smaller the singlet-triplet energy gap (ST gap), the stronger the radical character
We have shown that the TAO-LDA occupation numbers are qualitatively similar to the natural orbital occupation numbers (NOONs) obtained from the accurate RDM-CASSCF method, and are potentially useful for assessing the radical character of large alternant PAHs, due to its computational efficiency
Summary
We investigate the role of Kekulé and non-Kekulé structures in the radical character of alternant polycyclic aromatic hydrocarbons (PAHs) using thermally-assisted-occupation density functional theory (TAO-DFT), an efficient electronic structure method for the study of large ground-state systems with strong static correlation effects. To assess their radical character, Pelzer et al.[21] calculated the occupation numbers of the highest occupied (nHONO) and lowest unoccupied (nLUNO) natural orbitals for the lowest singlet states of these molecules, using the active-space variational two-electron reduced-density-matrix (RDM-CASSCF) method[43]. The structure with lower energy and greater degeneracy should have a larger weight, and govern the radical character of molecule α more significantly, while the structure with higher energy and less degeneracy should have a smaller weight, and govern the radical character of molecule α less significantly
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