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We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.

We describe the implementation and application of a recently developed time-dependent density-functional theory (TDDFT) algorithm based on the complex dynamical polarizability to calculate the photoabsorption spectrum of large metal clusters, with specific attention to the field of molecular plasmonics. The linear response TDDFT equations are solved in the space of the density fitting functions, so the problem is recast as an inhomogeneous system of linear equations whose resolution needs a numerical effort comparable to that of a SCF procedure. The construction of the matrix representation of the dielectric susceptibility is very efficient and is based on the discretization of the excitation energy, so such matrix is easily obtained at each photon energy value as a linear combination of constant matrix and energy-dependent coefficients. The code is interfaced to the Amsterdam Density Functional (ADF) program and is fully parallelized with standard message passing interface. Finally, an illustrative application of the method to the photoabsorption of the Au144(SH)60 cluster is presented.

Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Fundamental research on the covalent bonding of molecules with the surface can provide useful information on the organic/semiconductor interface. The NEXAFS spectroscopy represents a powerful technique to investigate the orientation and geometry of molecules adsorbed on surfaces and is widely used to characterize adsorbate structures, often in concert with theoretical calculations. The computational simulation of NEXAFS spectra of such systems represents a significant challenge both for a proper modelling of the adsorbate system as well as for the size of system which needs theoretical methods capable to fulfill requirements of accuracy and computational economy. Here we present a DFT-TS simulation of the NEXAFS spectra of pyridine adsorbed on a regular Si (100) surface, considering several adsorption models. The models have been previously optimized through periodic calculations performed with the Quantum Espresso code, then suitable finite clusters have been be cut out from the optimized periodic structures and used for the simulation of the angle resolved NEXAFS spectra of the adsorbed molecule, employing traditional molecular DFT techniques (through the ADF code). The results compare well with the experimental spectra and show that the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries and that the methodology employed is reliable to describe the K-shell spectra of this kind of systems.

Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Since the properties of the resulting materials are largely dependent on the organic/semiconductor interface, fundamental research on the covalent bonding of molecules with the surface can provide useful information. Problems that have been addressed include the structure of the resulting systems and spectroscopic measurements often in concert with theoretical calculations can assess the orientation and geometry of the molecular adsorbate. In this respect NEXAFS spectroscopy represents a powerful technique to investigate the interaction of molecules with a surface. The theoretical simulation of NEXAFS spectra of molecules adsorbed on a surface represents a significant challenge both for a proper modelling of the adsorbate system as well as for the size of system which needs theoretical methods capable to fulfill requirements of accuracy and computational economy. Here we present DFT simulations of NEXAFS spectra of pyridine adsorbed on a regular Si (100) surface, considering several adsorption models.1 The surface and the adsorbate models have been previously optimized through periodic calculations, then suitable finite clusters have been be cut out from the optimized periodic structures and used for the simulation of the angle resolved NEXAFS spectra of the adsorbed molecule. The spectra have been calculated employing a molecular DFT methodology based on the transition potential scheme in order to include the core hole relaxation effect.2 Both the N1s and C1s adsorption edges of the adsorbed pyridine have been investigated. The results show that a careful analysis of the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries; the reliability of the computational strategy is proved by the comparison of the theoretical results with the experimental data.1,3 The periodic optimization of the molecules adsorbed on the surface plays a fundamental role in the design of clusters which correctly model the adsorbed system.

Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Since the properties of the resulting materials are largely dependent on the organic/semiconductor interface, fundamental research on the covalent bonding of molecules with the surface can provide useful information. Problems that have been addressed include the structure of the resulting systems and spectroscopic measurements often in concert with theoretical calculations can assess the orientation and geometry of the molecular adsorbate. NEXAFS spectroscopy is widely used to characterize adsorbate structures on surfaces since it allows to investigate the adsorption mode as well as the extent of the adsorbate-substrate interaction, through the comparison between the spectra of the free and adsorbed molecule. Here we present a DFT simulation of the NEXAFS spectra of ethylene adsorbed on a regular Si (100) surface, considering several adsorption models. Cluster models have been employed to mimic the molecule-surface system for the spectra simulation. Surface models with and without the adsorbed molecule have been optimized by means of periodic slab DFT calculations. Both total and angle dependent spectra have been calculated and compared with the available polarization dependent NEXAFS experiments; the good agreement between theoretical and experimental results proves the reliability of the present computational strategy.

Theoretical simulation of the electronic spectra needs for computational approaches capable to correctly describe the excited states of the system. The time dependent density functional theory (TDDFT) allows the treatment of the electronic excited states and its relatively computational economy makes it a good candidate to treat large systems. X-Ray absorption Spectroscopy (XAS) has proven to be a powerful local probe technique for the investigation of both electronic and geometric structure of condensed matter. The computational strategy adopted to describe the XAS spectra of solid samples is based on the TDDFT method, properly extended to the computation of core electron excitations, and the finite size cluster model for the simulation of the bulk. Cluster model appears suitable for the calculation of core excitations since the excited electron is localized near the core hole therefore the description of a limited region would be sufficient to describe it accurately. Here we present an application of the TDDFT core scheme at the relativistic spin-orbit (SO) level to the simulation of the XAS spectra of bulk transition metal oxides V2O5 and TiO2 at the V and Ti L2,3 edges. It is well known that the SO coupling plays an important role in the spectral shapes of L3 and L2 edges of transition metal oxides and strongly influence their intensity distribution, giving rise to distinct spectral features converging to the L3 and L2 ionization thresholds. Calculations at this level of accuracy of the 2p metal edge absorption spectra of solid systems represent a novelty for condensed systems. The very good agreement between calculated and experimental features show the reliability of the method to take into account the complex effects involved in the spectroscopy of transition metal compounds where the correlation effects are important and the spin-orbit coupling large.

Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Since the properties of the resulting materials are largely dependent on the organic/semiconductor interface, fundamental research on the covalent bonding of molecules with the surface can provide useful information. Problems that have been addressed include the structure of the resulting systems and spectroscopic measurements often in concert with theoretical calculations can assess the orientation and geometry of the molecular adsorbate. NEXAFS spectroscopy is widely used to characterize adsorbate structures on surfaces since it allows to investigate the adsorption mode as well as the extent of the adsorbate-substrate interaction, through the comparison between the spectra of the free and adsorbed molecule. Here we present a DFT simulation of the NEXAFS spectra of ethylene adsorbed on a regular Si (100) surface, considering several adsorption models. Cluster models have been employed to mimic the molecule-surface system for the spectra simulation. Surface models with and without the adsorbed molecule have been optimized by means of periodic slab DFT calculations. Angle dependent spectra have been also calculated and compared with available polarization dependent NEXAFS experiments, revealing the suitability of this technique to derive the adsorption configuration of the molecule on the surface.

The time-dependent density functional theory approach has been employed to simulate the core excitation spectra of model systems for the adsorption of SO2 on the MgO (100) regular surface. All of the edges, sulfur K and L edges and oxygen K edge, have been considered in the calculations. Cluster models are adopted to describe the different interaction modes of SO2 With the acidic and basic sites of the surface. The main aspects of the MgO surface modeling have been considered, and their influence on the calculated spectra of the adsorbed molecule has been analyzed. The results show that the method provides a viable tool for obtaining excitation energies and oscillator strengths in this big system and allows for the investigation of the origin of the spectral features. The comparison between the spectra calculated for the adsorbate models and the spectrum of the free SO, allows for the discussion of the nature of the adsorbate-substrate interaction (physi- or chemisorption) in terms of the differences in their core excitation spectra.