Extensive benchmarks of exchange-correlation functionals on real X-ray resolved nanoclusters have been carried out and reported here for the first time. The systems investigated and used for the tests are two undecagold and one Au24+-based nanoclusters stabilized by thiol and phosphine ligands. Time-dependent density-functional theory has been used to compare calculations with experimental data on optical gaps. It has been observed that GGA functionals employing PBE-like correlation (viz., PBE itself, B-PBE, B-P86, and B-PW91) coupled with an improved version of the LANL2DZ pseudopotential and basis set provide accurate results for both the structure and optical gap of gold nanoclusters, at a reasonable computational cost. Good geometries have been also obtained using some global hybrid (e.g., PBE0, B3-P86, mPW1-PW91) and range-separated hybrid (e.g., HSE06) functionals making use of PBE-like correlation, even though they yield optical gaps overestimating the experimental findings up to 0.5 eV. Popular exchange-correlation combinations such as B-LYP and B3-LYP deform cluster geometry during structural optimization, probably due to the LYP correlation. Effects of the stabilizing organic ligands on the properties of metal cores have been probed simulating the nanoclusters at the density-functional level of theory retaining the organic coating. This paper provides a useful contribution to the simulations of structural and optoelectronic properties of larger metal–organic particles suitable for a wide range of nanotechnological applications

Time-dependent density functional theory (TD-DFT) is usually benchmarked by evaluating how the vertical excitation energies computed by using different exchange-correlation (XC) functionals compare with the maximum of the absorption spectra. However, the latter does not necessarily coincide with the vertical energies because it is affected by the vibronic band structure that has to be properly taken into account. In this work, we have evaluated the performance of several functionals belonging to different families in reproducing the vibronic structure (band shape) of four 7-aminocoumarin molecules of technological interest, whose spectra have been recorded in methylcyclohexane and acetonitrile solvents. In order to compare the computed vibronic spectra with the experimental ones in the most consistent way, the effect of temperature, often neglected, was also taken into account. We have found that no single functional provides simultaneously accurate band positions and shapes, but the combination of ωB97X vibronic couplings with PBE0 vertical energies can lead to very satisfactory results. In addition to the assessment of XC functionals, several adiabatic and vertical models proposed in the literature to compute vibrationally resolved electronic spectra have been tested and validated with respect to experiments. On these grounds, the adiabatic Hessian model has been used to perform a complete analysis of the ωB97X/PBE0 vibronic transitions contributing to the final band shapes of the investigated aminocoumarin molecules.

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The effect of silver doping on the electronic properties and photoluminescence of a class of structurally similar AgnAu25–n2+ nanoclusters (0 ≤ n ≤ 13) has been investigated here by means of time-dependent density functional calculations. As very recently reported in the literature, a mixture of these clusters showed an unexpected 200-fold fluorescence quantum yield boost with respect to Au252+, but no mechanism has been proposed to date to explain this phenomenon. The results presented here suggest that the origin of this boost lies in the nature of the first excited state (S1), which is affected differently by the increasing presence of Ag atoms into the network of Au atoms. In fact, doping the cluster with silver atoms has the effect of shifting the lowest-energy “dark” excited states to higher energy, leaving a very “bright” highest occupied molecular orbital → lowest unoccupied molecular orbital (HOMO → LUMO) transition as the lowest-energy excitation. We propose that when fluorescence occurs from “bright” S1 states, it receives a boost in the quantum yield because of the high oscillator strength of these HOMO → LUMO transitions

The H-bond interactions of methyl acetate in methanol have been studied by means of ab initio molecular dynamics simulations within the Car−Parrinello approach. It has been observed that the CO stretching band of methyl acetate splits into a doublet as a consequence of the interaction with the solvent. The H-bond effects on the spectroscopic properties of methyl acetate in methanol have been interpreted by wavelet transform analysis in conjunction with a structural and dynamic characterization of the solvation cage. Localizing a vibrational mode in time and frequency during the simulations has allowed association of the different interactions with the solvent to the vibrational properties. This represents an important development in the capability of molecular dynamics simulations to explain experimental data obtained by time-resolved spectroscopic methods.