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TDDFT simulations of the absorption and CD spectra of a Pd 2 Au 36 (SC 2 H 4 Ph) 24 monolayer-protected cluster (MPC) are carried out with the aim of investigating the effects of doping, conformational degrees of freedom of the thiolates' end-groups, and charge states on the optical and dichroic response of a prototypical MPC species. Clear signatures of Pd doping in both absorption and CD spectra are found to be a consequence of the participation of Pd (4d) states in the ligand-based d-band and on the unoccupied MOs of lower energy. Exploration of conformational space points to a much greater sensitivity of optical rotation to the conformation of the end-groups of the organic monolayer compared to absorption. Finally, the effect of charge is mainly seen as a decreased dependence of the dichroic response on conformation. The agreement between the TDDFT predictions and the available experimental data is good, and enables an assignment of absorption and CD bands to specific classes of one-particle excitations.

Au-38(SR)(24) is one of the most extensively investigated gold nanomolecules along with Au-25(SR)(18) and Au-144(SR)(60). However, so far it has only been prepared using aliphatic-like ligands, where R = -SC6H13, -SC12H25 and -SCH2CH2Ph. Au-38(SCH2CH2Ph)(24) when reacted with HSPh undergoes core-size conversion to Au-36(SPh)(24), and existing literature suggests that Au-38(SPh)(24) cannot be synthesized. Here, contrary to prevailing knowledge, we demonstrate that Au-38(SPh)(24) can be prepared if the ligand exchanged conditions are optimized, under delicate conditions, without any formation of Au-36(SPh)(24). Conclusive evidence is presented in the form of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectra (ESI-MS) characterization, and optical spectra of Au-38(SPh)(24) in a solid glass form showing distinct differences from that of Au-38(S-aliphatic)(24). Theoretical analysis confirms experimental assignment of the optical spectrum and shows that the stability of Au-38(SPh)(24) is not negligible with respect to that of its aliphatic analogous, and contains a significant component of ligand-ligand attractive interactions. Thus, while Au-38(SPh)(24) is stable at RT, it converts to Au-36(SPh)(24) either on prolonged etching (longer than 2 hours) at RT or when etched at 80 degrees C.

Here we investigate via first-principles simulations the optical absorption spectra of three different Au-30(SR)(18) monolayer-protected clusters (MPC): Au-30((SBu)-Bu-t)(18), Au-30(SPh)(18), and Au-30(SPh-pNO2)(18). Au-30((SBU)-B-t)(18) is known in the literature, and its crystal structure is available. In contrast, Au-30(SPh)(18) and Au-30(SPh-pNO(2))(18) are two species that have been designed by replacing the tertbutyl organic residues of Au-30((SBu)-Bu-t)(18) with aromatic ones so as to investigate the effects of ligand replacement on the optical response of Au nanomolecules. By analogy to a previously studied Au-23(SR)(16)(-) anionic species, despite distinct differences in charge and chemical composition, a substantial ligand enhancement of the absorption intensity in the optical region is also obtained for the Au-30(SPh-pNO(2))(18) MPC. The use of conjugated aromatic ligands with properly chosen electron-withdrawing substituents and exhibiting steric hindrance so as to also achieve charge decompression at the surface is therefore demonstrated as a general approach to enhancing the MPC photoabsorption intensity in the optical region. Additionally, we here subject the ligand-enhancement phenomenon to a detailed analysis based on the fragment projection of electronic excited states and on induced transition densities, leading to a better understanding of the physical origin of this phenomenon, thus opening avenues to its more precise control and exploitation.

We report the complete X-ray crystallographic structure as determined through single-crystal X-ray diffraction and a thorough theoretical analysis of the green gold Au-30(S-tBu)(18). While the structure of Au30S(S-tBu)(18) with 19 sulfur atoms has been reported, the crystal structure of Au-30(S-tBu)(18) without the mu(3)-sulfur has remained elusive until now, though matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS) data unequivocally show its presence in abundance. The Au-30(S-tBu)(18) nanomolecule not only is distinct in its crystal structure but also has unique temperature-dependent optical properties. Structure determination allows a rigorous comparison and an excellent agreement with theoretical predictions of structure, stability, and optical response.

TDDFT simulations of the absorption and CD spectra of a Pd2Au36(SC2H4Ph)24 monolayer-protected cluster (MPC) are carried out with the aim of investigating the effects of doping, conformational degrees of freedom of the thiolates' end-groups, and charge states on the optical and dichroic response of a prototypical MPC species. Clear signatures of Pd doping in both absorption and CD spectra are found to be a consequence of the participation of Pd (4d) states in the ligand-based d-band and on the unoccupied MOs of lower energy. Exploration of conformational space points to a much greater sensitivity of optical rotation to the conformation of the end-groups of the organic monolayer compared to absorption. Finally, the effect of charge is mainly seen as a decreased dependence of the dichroic response on conformation. The agreement between the TDDFT predictions and the available experimental data is good, and enables an assignment of absorption and CD bands to specific classes of one-particle excitations.

A validation study of the computational protocol for predicting the optical properties of Monolayer-Protected metal Clusters (MPCs) using time-dependent density-functional theory is presented. The Au25(SC2H4Ph)18- MPC is chosen as a reference, due to the availability of both structural and optical experimental data. The effects of the geometry, the basis set, the exchange-correlation functionals, and the use of simplified or experimental ligands on the optical properties of Au25(SC2H4Ph)18- are discussed critically. When such options are carefully selected, an almost quantitative matching between theory and experiment is obtained. Noteworthy, the use of a precise geometric structure proves to be both crucial and critical for an accurate prediction of the optical response of MPC systems, a feature which is not easy to achieve using current density-functional theory approaches.

We introduce individual component maps of rotatory strength (ICM-RS) and rotatory strength density (RSD) plots as analysis tools of chiro-optical linear response spectra deriving from time-dependent density functional theory (TDDFT) simulations. ICM-RS and RSD allow one to visualize the origin of chiro-optical response in momentum or real space, including signed contributions and therefore highlighting cancellation terms that are ubiquitous in chirality phenomena, and should be especially useful in analyzing the spectra of complex systems. As test cases, we use ICM-RS and RSD to analyze circular dichroism spectra of selected (Ag− Au)30(SR)18 monolayer-protected metal nanoclusters, showing the potential of the proposed tools to derive insight and understanding, and eventually rational design, in chiro-optical studies of complex systems.

The Near-Edge X-ray-Absorption Fine-Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) of short-chain oligothiophenes (thiophene, 2,2'-bithiophene, and 2,2':5',2 ''-terthiophene) in the gas phase have been measured in the sulfur L-2,L-3-edge region. The assignment of the spectral features is based on the relativistic two-component zeroth-order regular approximation time dependent density functional theory approach. The calculations allow us to estimate both the contribution of the spin-orbit splitting and of the molecular-field splitting to the sulfur binding energies and give results in good agreement with the experimental measurements. The deconvolution of the calculated S2p NEXAFS spectra into the two manifolds of excited states converging to the L-III and L-II edges facilitates the attribution of the spectral structures. The main S2p NEXAFS features are preserved along the series both as concerns the energy positions and the nature of the transitions. This behaviour suggests that the electronic and geometrical environment of the sulfur atom in the three oligomers is relatively unaffected by the increasing chain length. This trend is also observed in the XPS spectra. The relatively simple structure of S2p NEXAFS spectra along the series reflects the localized nature of the virtual states involved in the core excitation process. Published by AIP Publishing.

Au

The electronic structure of short-chain thiophenes (thiophene, 2,2 '-bithiophene, and 2,2 ': 5 ',2 ''-terthiophene) in the gas phase has been investigated by combining the outcomes of Near-Edge X-ray-Absorption Fine-Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) at the C K-edge with those of density functional theory (DFT) calculations. The calculated NEXAFS spectra provide a comprehensive description of the main experimental features and allow their attribution. The evolution of the C1s NEXAFS spectral features is analyzed as a function of the number of thiophene rings; a tendency to stabilization for increasing chain length is found. The computation of the binding energy allows to assign the experimental XPS peaks to the different carbon sites on the basis of both the inductive effects generated by the presence of the S atom as well as of the differential aromaticity effects. Published by AIP Publishing.

We apply a recently developed time-dependent density functional theory (TDDFT) algorithm based on the complex dynamical polarizability to calculate the photoabsorption spectrum of the following series of closed-shell icosahedral clusters of increasing size (namely, [M13]5+, [M55]3-, [M147]-, and [M309]3+ with M = Ag, Au), focusing in particular on their plasmonic response. The new method is shown to be computationally very efficient: it simultaneously retains information on the excited-state wave function and provides a detailed analysis of the optical resonances, e.g., by employing the transition contribution map scheme. For silver clusters, a very intense plasmon resonance is found for [Ag55]3-, with strong coupling among low-energy single-particle configurations. At variance, for gold clusters we do not find a single strong plasmonic peak but rather many features of comparable intensity, with partial plasmonic behavior present only for the lowest-energy transitions. Notably, we also find a much greater sensitivity of the optical response of Ag clusters with respect to Au clusters to cluster charge, the exchange-correlation (xc) functional, and the basis set, as demonstrated via a detailed comparison between [Ag55]q and [Au55]q. The results of the TDDFT algorithm obtained with the complex dynamical polarizability are finally compared with those produced by alternative (real-time evolution or Lanczos) approaches, showing that, upon proper choice of numerical parameters, overall nearly quantitative agreement is achieved among all of the considered approaches, in keeping with their fundamental equivalence.

We detail the calculation of rotatory strengths as an extension of the complex polarizability algorithm of time dependent density functional theory (TDDFT) proposed in a recent publication (O. Baseggio, G. Fronzoni, M. Stener, J. Chem. Phys. 2015, 143, 024106). To demOnstrate the generality and applicability of the proposed algorithm, we calculate the photoabsorption and circular dichroism (CD) spectra of Ag-8 (as a validation case), Au-38(SCH3)(24), and Au-38(SCH2CH2Ph)(24) monolayer-protected gold clusters (as a system of great current interest for their optical properties). For Au-38(SCH2CH2Ph)(24), the computed CD spectrum agrees well with the experimental data from the literature. Furthermore, a comparison of the calculated CD spectra of the two thiolate-protected nanoclusters reveals that the most distinctive features of the spectra are rather insensitive to the nature of the thiolate tail groups, which, however, play a significant role in shaping optical and dichroic response of the systems, especially in the higher energy portion of the spectrum.

The electronic structures of some polycyclic aromatic hydrocarbons (PAHs) (antracene, phenantrene, cyclopentaphenantrene, coronene, crysene and tetracene) have been studied by means of synchrotron radiation spectroscopies at the carbon K-edge, XPS (X-ray Photoelectron spectroscopy) and NEXAFS (Near Edge X-ray Absorption Fine Structure), experimental data have been interpreted by comparison with theoretical calculations. NEXAFS spectra have been calculated at the spin-restricted DFT (Density Functional Theory) level. The core hole at each nonequivalent carbon center is modeled by the half core hole (also referred as the Transition Potential, TP) approximation. Intensity is calculated as electric dipole transition moment. Carbon 1s photoelectron spectra have been assigned by comparison with theoretical calculations. But it is also possible to unveil important discrepancies between theory and experimental data, which can be ascribed to electronic correlation effects or to vibronic coupling. Vibronic coupling effects have been explicitely studied in the case of the C K-edge NEXAFS of phenantrene and coronene [1]. Their inclusion even within the Franck-Condon approximation appear to improve considerably the agreement between theory and experiment[2]. The comparison with NEXAFS data from PAH deposited on surfaces, demonstrate that the vibronic coupling is essential for the correct interpretation of data also in the solid state [3,4]. References [1] Giovanna Fronzoni, Oscar Baseggio, Mauro Stener, Weijie Hua, Guangjun Tian, Yi Luo, Barbara Apicella; Michela Alfé, Monica de Simone, Antti Kivimäki, Marcello Coreno, J. Chem. Phys. 141 (2014) 044313; doi: 10.1063/1.4891221. [2] J. Lüder, M. de Simone, R. Totani, M. Coreno, C. Grazioli, B. Sanyal, O. Eriksson, B. Brena, C. Puglia J. Chem. Phys. 142 (2015) 074305; doi: 10.1063/1.4907723 [3] H. Oji, R. Mitsumoto, E. Ito, Y. Ouchi, K. Seki, T. Yokoyama, N. Kosugi J.Chem.Phys. 109 (1998) 10409 [4] F. Roth , J. Bauer, B. Mahns, B. Buchner Phys.Rev.B 85 (2012) 014513

A new algorithm to solve the Time Dependent Density Functional Theory (TDDFT) equations in the space of the density fitting auxiliary basis set has been developed and implemented. The method extracts the spectrum from the imaginary part of the polarizability at any given photon energy, avoiding the bottleneck of Davidson diagonalization. The original idea which made the present scheme very efficient consists in the simplification of the double sum over occupied-virtual pairs in the definition of the dielectric susceptibility, allowing an easy calculation of such matrix as a linear combination of constant matrices with photon energy dependent coefficients. The method has been applied to very different systems in nature and size (from H2 to [Au147](-)). In all cases, the maximum deviations found for the excitation energies with respect to the Amsterdam density functional code are below 0.2 eV. The new algorithm has the merit not only to calculate the spectrum at whichever photon energy but also to allow a deep analysis of the results, in terms of transition contribution maps, Jacob plasmon scaling factor, and induced density analysis, which have been all implemented.

We performed a combined experimental and theoretical study of the C1s Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy and X-ray Photoelectron Spectroscopy in the gas phase of two polycyclic aromatic hydrocarbons (phenanthrene and coronene), typically formed in combustion reactions. In the NEXAFS of both molecules, a double-peak structure appears in the C1s -> LUMO region, which differ by less than 1 eV in transition energies. The vibronic coupling is found to play an important role in such systems. It leads to weakening of the lower-energy peak and strengthening of the higher-energy one because the 0 - n (n > 0) vibrational progressions of the lower-energy peak appear in nearly the same region of the higher-energy peak. Vibrationally resolved theoretical spectra computed within the Frank-Condon (FC) approximation and linear coupling model agree well with the high-resolution experimental results. We find that FC-active normal modes all correspond to in-plane vibrations. (C) 2014 AIP Publishing LLC.

The electronic structures of fivepolycyclic aromatic hydrocarbons (PAHs) (antracene, fluorene, phenantrene, cyclopentaphenantrene and coronene) have been studied by means of synchrotron radiation spectroscopies at the carbon K-edge, XPS (X-ray Photoelectron spectroscopy) and NEXAFS (Near Edge X-ray Absorption Fine Structure), experimental data have been interpreted by comparison with theoretical calculations. NEXAFS spectra have been calculated with the Transition Potential (TP) scheme in the framework of the Density Functional Theory (DFT). Namely the Kohn-Sham equations are solved employing a basis set of Slater Type Orbitals with an occupation number of the C1s core orbital reduced by 0.5 electrons. Intensity is calculated as electric dipole transition moment. Carbon 1s photoelectron spectra have been assigned by comparison with theoretical calculations. But it is also possible to unveil important discrepancies between theory and experimental data, which can be ascribed to electronic correlation effects or to vibronic coupling. Vibronic coupling effects have been explicitely studied in the case of the C K-edge NEXAFS of phenantrene and coronene. Their inclusion even within the Franck-Condon approximation appear to improve considerably the agreement between theory and experiment.

Quantum ESPRESSO is an open-source distribution of computer codes for quantum-mechanical materials modeling, based on density-functional theory, pseudopotentials, and plane waves, and renowned for its performance on a wide range of hardware architectures, from laptops to massively parallel computers, as well as for the breadth of its applications. In this paper, we present a motivation and brief review of the ongoing effort to port Quantum ESPRESSO onto heterogeneous architectures based on hardware accelerators, which will overcome the energy constraints that are currently hindering the way toward exascale computing.