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Title: | Optical excitations of defects in realistic nanoscale silica clusters: comparing the performance of density functional theory using hybrid functionals with correlated wavefunction methods |
Author: | Zwijnenburg, M. A. Sousa Romero, Carmen Sokol, A. A. Bromley, Stefan Thomas |
Keywords: | Isomeria Optimització matemàtica Defectes cristal·lins Teoria del funcional de densitat Silicats Propietats òptiques Isomerism Mathematical optimization Crystals defects Density functionals Silicates Optical properties |
Issue Date: | 3-Jul-2008 |
Publisher: | American Institute of Physics |
Abstract: | Optical excitations of low energy silica SiO24 clusters obtained by global optimization, as opposed to constructed by hand, are studied using a range of theoretical methods. By focusing on the lowest energy silica clusters we hope to capture at least some of the characteristic ways by which the dry surfaces of silica nanosystems preferentially terminate. Employing the six lowest energy SiO24 cluster isomers, we show that they exhibit a surprisingly wide range of geometries, defects, and associated optical excitations. Some of the clusters show excitations localized on isolated defects, which are known from previous studies using hydrogen-terminated versions of the defect in question. Other clusters, however, exhibit novel charge-transfer excitations in which an electron transfers between two spatially separated defects. In these cases, because of the inherent proximity of the constituent defects due to the small cluster dimensions, the excitation spectrum is found to be very different from that of the same defects in isolation. Excitation spectra of all clusters were calculated using time-dependent density functional theory TD-DFT and delta-SCF DFT DFT methods employing two different hybrid density functionals B3LYP and BB1K differing essentially in the amount of incorporated Hartree-Fock-like exchange HFLE. In all cases the results were compared with CASPT2 calculated values which are taken as a benchmark standard. In line with previous work, the spatially localized excitations are found to be well described by TD-DFT/B3LYP but which gives excitation energies that are significantly underestimated in the case of the charge-transfer excitations. The TD-DFT/BB1K combination in contrast is found to give generally good excitation energies for the lowest excited states of both localized and charge-transfer excitations. Finally, our calculations suggest that the increased quality of the predicted excitation spectra by adding larger amounts of HFLE is mainly due to an increased localization of the excited state associated with the elimination of spurious self-interaction inherent to semi-local DFT functionals. |
Note: | Reproducció del document publicat a: https://doi.org/10.1063/1.2943147 |
It is part of: | Journal of Chemical Physics, 2008, vol. 129, num. 1, p. 014706-1-014706-7 |
URI: | http://hdl.handle.net/2445/150677 |
Related resource: | https://doi.org/10.1063/1.2943147 |
ISSN: | 0021-9606 |
Appears in Collections: | Articles publicats en revistes (Ciència dels Materials i Química Física) |
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