Optical excitations of low-energy silica (SiO2)4 clusters obtained by global optimization, as opposed to constructed by hand, were studied by using a range of theoretical methods. By focusing upon the lowest-energy silica clusters it was hoped to capture some of the characteristic ways in which the dry surfaces of silica nanosystems preferentially terminated. Using the 6 lowest-energy (SiO2)4 cluster isomers, it was shown that they exhibited a surprisingly wide range of geometries, defects and associated optical excitations. Some of the clusters exhibited excitations localized on isolated defects, which were known from previous studies using H-terminated versions of the defect in question. However, other clusters exhibited novel charge-transfer excitations in which an electron transferred between 2 spatially separated defects. In these cases, because of the inherent proximity of the constituent defects due to the small cluster dimensions, the excitation spectrum was found to be very different from that of the same defects in isolation. The excitation spectra of all clusters were calculated by using time-dependent density functional theory (TD-DFT) and delta-SCF density functional theory (ΔDFT) methods involving 2 different hybrid density functionals (B3LYP and BB1K) differing essentially in the amount of incorporated Hartree–Fock-like exchange. In each case, the results were compared with CASPT2 calculated values which were taken as a benchmark standard. The spatially localized excitations were found to be well described by TD-DFT/B3LYP but gave excitation energies that were significantly underestimated in the case of the charge-transfer excitations. The TD-DFT/BB1K combination was found to give generally good excitation energies for the lowest excited states of both localized and charge-transfer excitations. The calculations suggested that the increased quality of the predicted excitation spectra by adding larger amounts of Hartree–Fock-like exchange was due mainly to an increased localization of the excited state associated with the elimination of the spurious self-interaction which was inherent to (semi-)local density functional theory functionals.

Optical Excitations of Defects in Realistic Nanoscale Silica Clusters - Comparing the Performance of Density Functional Theory Using Hybrid Functionals with Correlated Wavefunction Methods. M.A.Zwijnenburg, C.Sousa, A.A.Sokol, S.T.Bromley: Journal of Chemical Physics, 2008, 129[1], 014706