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We analyse the spinterface formed by a C60 molecular layer on a Fe(001) surface covered by a two-dimensional Cr4O5 layer. We consider different geometries, by combining the high symmetry adsorption sites of the surface with three possible orientations of the molecules in a fully relaxed Density Functional Theory calculation. We show that the local hybridization between the electronic states of the Cr4O5 layer and those of the organic molecules is able to modify the magnetic coupling of the Cr atoms. Both the intra-layer and the inter-layer magnetic interaction is indeed driven by O atoms of the two-dimensional oxide. We demonstrate that the C60 adsorption on the energetically most stable site turns the ferromagnetic intra-layer coupling into an antiferromagnetic one, and that antiferromagnetic to ferromagnetic switching and spin patterning of the substrate could be possible by adsorption on other sites.

We predict the optical properties (in particular optical absorbance and reflectance anisotropy spectra (RAS)) of the silicon(001) surface organically functionalized by the adsorption of chosen tetr...

Hybrid sp-sp2 structures can be efficiently obtained on metal substrates via on-surface synthesis. The choice of both the precursor and of the substrate impacts on the effectiveness of the process and the stability of the formed structures. Here we demonstrate that using anthracene-based molecules as precursor, the formation on Au(111) of polymers hosting sp carbon chains is affected by the steric hindrance between aromatic groups. In particular, by scanning tunneling microscopy and density functional theory calculations we show that the de-metalation of organometallic structures induces a lateral separation of adjacent polymers preventing the formation of ordered domains.

In this work, we study the optical absorption spectra of extensive stacking forms of four molecules known to be among eumelanin basic building blocks: 5,6-dihydroxyindole (DHI), 5,6-dihydroxyindole-2-carboxylic acid (DHICA), indolequinone (IQ) and quinone-methide (MQ). Stacked monomers can be considered as a minimal model of out-of-plane complexity in eumelanin, as complementary to in-plane oligomerization. The choice of a plane wave density functional theory (DFT) approach allows us to address extensive, i.e. several-layer, stacking of these DHI-like monomers at a relatively low computational cost, by treating stacked systems as periodic along the stacking direction. Absorption spectra of stacked monomers display interesting trends in terms of chemical and structural sensitivity, which can shed light on the role of extensive stacking in the transition from the optical properties of small molecules such as DHI to the typical broadband and monotonic absorption spectrum of the eumelanin pigment.

We have computed from first principles the structural, magnetic and optical properties of zig-zag oriented silicene nanoribbons. The minimal width for structurally stable planar structure having zig-zag edges corresponds to a 4-chains ribbon. Its ground state presents reconstructed and spin polarized edges, coupled antiferromagnetically. For this state, and for the corresponding excited state with ferromagnetically coupled edges, we computed the optical absorption spectra within the independent particle approximation, including local field effect corrections, for light polarized in the directions parallel and perpendicular to the ribbon axis. For the “parallel” light polarization the inclusion of local fields effects is limited to a slight reduction of the intensity of the main peak in the infrared region, as well as that of some minor peaks in the visible-ultraviolet structure. Conversely, the computed optical spectrum for light polarized perpendicularly to the ribbon axis shows that the short-width zig-zag nanoribbons are basically transparent in the infrared and visible region, because of the effect of electronic confinement combined with local fields. Keywords: Silicene, Optoelectronic properties, First principles simulations, 2D materials, Optical spectra, Nanoribbons.

The ability to accurately measure or predict several physicochemical properties of molecules which play a role as active substances in drugs can be of strategic importance for pharmacological applications, in addition to its possible interest in fundamental research. Chirality is a relevant feature in the characterization of drug molecules: enantiomers can show different pharmacological activity and adverse effects. The ability to separate stereoisomers and to assign their absolute configuration can thus be crucial. Circular dichroism (CD) spectra are a useful tool to distinguish between enantiomers. In this work we apply an in-house developed code, based on an efficient DFT approach for circular dichroism, to fully characterize the molecular optical properties in the case of few selected fundamental molecules for current medical and pharmaceutical research, namely avibactam, as representative of non β-lactam inhibitors, two cephems (cefepime and cefoxitin), as examples of β-lactam antibiotics, and idelalisib, as a recent relevant anticancer active substance to treat major leukemias. For the above molecules, in addition to their optical absorption spectra, we calculate their CD spectra within state-of-the-art computational techniques. We then investigate both the conformational and chemical sensitivity of absorption and CD spectra for the chosen molecules. The outcomes of the present research could be of fundamental importance to gain additional information on molecules involved in therapeutic protocols for severe diseases or in drug design.

We analyse the spinterface formed by a C

Special point defects in semiconductors have been envisioned as suitable components for quantum-information technology. The identification of new deep centers in silicon that can be easily activated and controlled is a main target of the research in the field. Vacancy-related complexes are suitable to provide deep electronic levels but they are hard to control spatially. With the spirit of investigating solid state devices with intentional vacancy-related defects at controlled position, here we report on the functionalization of silicon vacancies by implanting Ge atoms through single-ion implantation, producing Ge-vacancy (GeV) complexes. We investigate the quantum transport through an array of GeV complexes in a silicon-based transistor. By exploiting a model based on an extended Hubbard Hamiltonian derived from ab-initio results we find anomalous activation energy values of the thermally activated conductance of both quasi-localized and delocalized many-body states, compared to conventional dopants. We identify such states, forming the upper Hubbard band, as responsible of the experimental sub-threshold transport across the transistor. The combination of our model with the single-ion implantation method enables future research for the engineering of GeV complexes towards the creation of spatially controllable individual defects in silicon for applications in quantum information technologies.

Graphdiynes (GDYs) represent a class of 2D carbon materials based on sp-sp$^2$ hybridization with appealing properties and potential applications. Recent advances have demonstrated the experimental self-assembly of GDYs on metal substrates. Here we focus on $\alpha$- and $\beta$-GDYs on Au(111) and Pt(111), and investigate how their electronic and vibrational properties are affected by the interaction with a metal substrate. We adopt hydrogenated GDY, previously characterized experimentally, as a benchmark for density functional theory simulations, that we apply to show that Au and Pt substrates impose a different degree of distortion on both $\alpha$- and $\beta$-GDY. By comparing the adsorbed and the freestanding structures, we evaluate the effect of the surface interaction on the bandstructure and the simulated Raman spectra. Different charge transfers result in different energy shift of the Dirac cone in semi-metallic $\alpha$-GDY and changes from semiconducting to metallic behavior for $\beta$-GDY. These changes in electronic properties are accompanied by characteristic frequency shifts and modifications of Raman active modes. Our results contribute in the understanding of the metal-interaction effects on GDYs and can open a route to the design of novel 2D materials with tailored properties.

We analyse the spinterface formed by a C60 molecular layer on a Fe(001) surface covered by a two-dimensional Cr4O5 layer. We consider different geometries, by combining the high symmetry adsorption sites of the surface with three possible orientations of the molecules in a fully relaxed Density Functional Theory calculation.We show that the local hybridization between the electronic states of the Cr4O5 layer and those of the organic molecules is able to modify the magnetic coupling of the Cr atoms. Both the intra-layer and the inter-layer magnetic interaction is indeed driven by O atoms of the two-dimensional oxide. We demonstrate that the C60 adsorption on the energetically most stable site turns the ferromagnetic intra-layer coupling into an antiferromagnetic one, and that antiferromagnetic to ferromagnetic switching and spin patterning of the substrate are made possible by adsorption on other sites.

Long linear carbon nanostructures based on sp-hybridization can be synthesized by exploiting on-surface synthesis of halogenated precursors evaporated on Au(111), thus opening a way to investigations by surface-science techniques. By means of an experimental approach combining scanning tunneling microscopy and spectroscopy (STM and STS) with ex situ Raman spectroscopy we investigate the structural, electronic and vibrational properties of polymeric sp-sp2 carbon atomic wires composed by sp-carbon chains connected through phenyl groups. Density-functional-theory (DFT) calculations of the structure and the electronic density of states allow us to simulate STM images and to compute Raman spectra. The comparison of experimental data with DFT simulations unveil the properties and the formation stages as a function of the annealing temperature. Atomic-scale structural information from STM complement the Raman sensitivity to the single molecular bond to open the way to detailed understanding of these novel carbon nanostructures.

We present first-principles calculations of structural, electronic, magnetic, and optical properties of zigzag-oriented silicene nanoribbons, which, being endowed with spin-polarized edge states, are promising candidates as building blocks of future spintronic devices. The minimal width for a structurally stable planar structure having zigzag edges corresponds to a 4-chain ribbon, whose ground state presents antiferromagnetically coupled spin-polarized edges, and a lattice parameter along the nanoribbon axis contracted ($\ensuremath{\sim}5%$) with respect to the bulk value. Starting from the dependence of structural and electronic properties on the ribbon width, we present theoretical predictions for the optical spectra of narrow nanoribbons, in which excitonic effects are relevant due to the confinement in a quasi-one-dimensional structure. Especially for light polarized parallel to the ribbon axis, we find significant differences in the position of optical absorption peaks of ribbons with ferro- or antiferromagnetically coupled edges, showing that optical spectra can be used as a fingerprint of the magnetic coupling of electronic edge states.

The need of a fs-scale pulsed, high repetition rate, X-ray source for time-resolved fine analysis of matter (spectroscopy and photon scattering) in the linear response regime is addressed by the conceptual design of a facility called MariX (Multi-disciplinary Advanced Research Infrastructure for the generation and application of X-rays) outperforming current X-ray sources for the declared scope. MariX is based on the original design of a two-pass two-way superconducting linear electron accelerator , equipped with an arc compressor, to be operated in CW mode (1 MHz). MariX provides FEL emission in the range 0.2–8 keV with 1 0 8 photons per pulse ideally suited for photoelectric effect and inelastic X-ray scattering experiments. The accelerator complex includes an early stage that supports an advanced inverse Compton source of very high-flux hard X-rays of energies up to 180 keV that is well adapted for large area radiological imaging, realizing a broad science programme and serving a multidisciplinary user community, covering fundamental science of matter and application to life sciences, including health at preclinical and clinical level.

Long linear carbon nanostructures based on sp-hybridization can be synthesized by exploiting on-surface synthesis of halogenated precursors evaporated on Au(111), thus opening a way to investigations by surface-science techniques. By means of an experimental approach combining scanning tunneling microscopy and spectroscopy (STM and STS) with ex situ Raman spectroscopy we investigate the structural, electronic and vibrational properties of polymeric sp–sp2 carbon atomic wires composed by sp-carbon chains connected through phenyl groups. Density-functional-theory (DFT) calculations of the structure and the electronic density of states allow us to simulate STM images and to compute Raman spectra. The comparison of experimental data with DFT simulations unveil the properties and the formation stages as a function of the annealing temperature. Atomic-scale structural information from STM complement the Raman sensitivity to the single molecular bond to open the way to detailed understanding of these novel carbon nanostructures.

Carbon structures comprising sp 1 chains (e.g., polyynes or cumulenes) can be synthesized by exploiting on-surface chemistry and molecular self-assembly of organic precursors, opening to the use of the full experimental and theoretical surface-science toolbox for their characterization. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) can be used to determine molecular adsorption angles and is here also suggested as a probe to discriminate sp 1 /sp 2 character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp 1 /sp 2 materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp 2 systems. We then consider a mixed polymer alternating sp 1 C 4 units with sp 2 biphenyl groups, recently synthesized on Au(111), as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the the presence of linear C chains. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp 1 hybridization in surface-grown C structures.

We characterize germanium-vacancy GeVn complexes in silicon using first-principles Density Functional Theory calculations with screening-dependent hybrid functionals. We report on the local geometry and electronic excited states of these defects, including charge transition levels corresponding to the addition of one or more electrons to the defect. Our main theoretical result concerns the GeV complex, which we show to give rise to two excited states deep in the gap, at -0.51 and -0.35 eV from the conduction band, consistently with the available spectroscopic data. The adopted theoretical scheme, suitable to compute a reliable estimate of the wavefunction decay, leads us to predict that such states are associated to an electron localization over a length of about 0.45 nm. By combining the electronic properties of the bare silicon vacancy, carrying deep states in the band gap, with the spatial controllability arising from single Ge ion implantation techniques, the GeVn complex emerges as a suitable ingredient for silicon-based room-temperature single-atom devices.

We present two test cases showing how electronic and optical spectroscopy techniques and their theoretical understanding by first principles can provide information on the strength of the adsorbate-substrate electronic interaction at interfaces. Results of electronic band structure calculations for two-dimensional silicon sheets (“silicene”) grown on silver are shown, pointing out that the interaction with the metal substrate is strong enough as to disrupt many of the exceptional properties computed for the free-standing material, or to mitigate the differences between various two-dimensional allotropes. The optical properties of silicon surface functionalized by nucleobase molecules are then discussed. Calculations predict that chemical sensitivity of spectra to molecular modifications can be overwhelmed by variations in the substrate response as induced by the adsorbates.

In this work, we predict and analyze the optical spectra of pyrimidinic uracil-like nucleobases thymine (THY), uracil (URA), and 5-fluorouracil (5-FU) and their reflection anisotropy spectra (RAS) upon adsorption on the silicon (001) surface. First-principles results based on plane-wave density functional theory show chemically sensitive features in gas phase optical absorption spectra that redshift/blueshift according to the orbitals involved in the corresponding transition. In the RAS, a characteristic lineshape is found, typical of the energetically favored "dimer bridge" configuration, and remarkably similar for all the investigated Si(001):X systems (X = THY, URA, 5-FU). We show that molecular tilting and breaking of the glide plane symmetry have a negligible effect on the optical spectra, despite their influence on the surface bandstructure. Contrarily to gas phase spectra, chemically sensitive RAS features only appear above 4.5 eV, and can be recognized as molecular contributions consistent with gas phase optical absorption results whereas substrate effects dominate at lower energies. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Starting with an introductory overview of the relevant aspects that characterize single-electron transport via individual dopant atom in silicon, we briefly discuss the role of theoretical calculations at different levels of approximation, as supporting tool for the interpretation of the experiments. In particular we suggest to adopt Density Functional Theory with screened exchange hybrid potential as suitable formalism to achieve an accurate description of the local geometry and excited states electronic structure of dopant-related defects. We present the results obtained for the test case of a single vacancy in silicon considering the band structure as benchmark to appreciate the advantages of the method. A complete characterization of the dopant-related levels in the gap would require to evaluate the charge transition state levels. This calculation paves the way towards a systematic study of more complex defects within the same theoretical scheme, including systems of applicative interest.

We study the conformational dependence of circular dichroism (CD) spectra of amino acid molecules by means of an efficient ab initio DFT approach which is free from the typical gauge invariance issues arising with the use of localized basis sets and/or real-space grids. We analyze the dependence of the chiroptical spectra on the backbone dihedrals in the specific case of alanine and consider the role of side chain degrees of freedom at the examples of leucine, phenylalanine, and serine, whose side chains have different physicochemical properties. The results allow one to identify the most diagnostic regions of the CD spectra and to critically compare the conformations which match the experimental CD data with conformations extracted from the rotamer library. The inclusion of a solvation shell of explicit water molecules and its effect on the CD spectrum are analyzed at the example of alanine.

The recent integration of silicene in field-effect transistors (FET) opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional allotropic form of silicon. Intense efforts have been devoted to the study of the epitaxial Silicene/Ag(111) system in order to elucidate the presence of Dirac fermion in analogy with graphene; strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of π and π* bands. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility, points out to a complex physics at the silicene-silver interface, demanding for a deeper comprehension of its details on the atomic scale. Here we elucidate the role of the metallic support in determining the physical properties of the Si/Ag interface, by means of optical techniques combined with theoretical calculations of the optical response of the supported system. The silicene/Ag(111) spectra, which turn out to be strongly non-additive, are analyzed in the framework of theoretical density functional based calculations allowing us to single out contributions arising from different localization. Electronic transitions involving silver states are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The results point to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of Transient-Reflectance spectroscopy. The latter shows a metallic-like carrier dynamics, (both for silicene and amorphous silicon), hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems.

Silicene, a honeycomb lattice of silicon, presents a particular case of allotropism on Ag(111). Silicene forms multiple structures with alike in-plane geometry but different out-of-plane atomic buckling and registry to the substrate. Angle-resolved photo emission and first-principles calculations show that these silicene structures, with (4x4), (root 13x root 13)R13.9 degrees, and (2 root 3x2 root 3)R30 degrees lattice periodicity, display similar electronic bands despite the structural differences. In all cases the interaction with the substrate modifies the electronic states, which significantly differ from those of free-standing silicene. Complex photoemission patterns arise from surface umklapp processes, varying according to the periodicity of the silicene allotropes.

We study the electronic properties of the Si(001):Uracil, Si(001):Thymine, and Si(001):5-Fluorouracil systems, focusing on the Si dimer-bridging configuration with adsorption governed by carbonyl groups. While the overall structural and electronic properties are similar, with small differences due to chemical substitutions, much larger effects on the surface band dispersion and bandgap show up as a function of the molecular orientation with respect to the surface. An off-normal orientation of the molecular planes is favored, showing larger bandgap and lower total energy than the upright position. We also analyze the localization of gap-edge occupied and unoccupied surface states.

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$., Comment: Accepted for publication in J. Phys. Chem. C

We compute high-resolution electron energy loss spectra (HREELS) of possible structural motifs that form during the dynamic oxidation process on Si(100), including the important metastable precursor silanone and an adjacent-dimer bridge (ADB) structure that may seed oxide formation. Spectroscopic fingerprints of single site, silanone, and "seed" structures are identified and related to changes in the surface bandstructure of the clean surface. Incorporation of oxygen into the silicon lattice through adsorption and dissociation of water is also examined. Results are compared to available HREELS spectra and surface optical data, which are closely related. Our simulations confirm that HREELS offers complementary evidence to surface optical spectroscopy, and show that its high sensitivity allows it to distinguish between energetically and structurally similar oxidation models.

The European Theoretical Spectroscopy Facility (ETSF) is a distributed knowledge network that gives researchers access to state-of-the-art computer simulations for electronic excited states in matter. Focusing on the fundamental knowledge of matter at the quantum-mechanical level, ETSF seeks to transfer this understanding to the future design of technologies in multiple areas.

Electron energy loss spectra of the Si(100) and Si(100):O surfaces are investigated using ab initio methods based on density functional theory. Computed spectra for the clean surface are found to be in good agreement with published experimental data, providing further confirmation that reconstructions of higher order than (2 x 1) are present on Si(100). The origins of the main spectral structures are analyzed and their relation to the surface bandstructure is illustrated. Oxygen adsorption on the surface dimers is found to cause strong modifications to the calculated spectra, thereby partially explaining the results of surface differential reflectivity experiments.

We present an ab initio investigation of the optical properties of 3-tert-butylcyclohexene in both its conformers. The optical spectra, here the photoabsorption cross section, have been obtained within density-functional theory at the independent-particle level, and within time-dependent density-functional theory. The optical spectra of the two conformers show small but visible differences, hence suggesting that optical absorption experiments can discriminate among the two molecular geometries. To cite this article: K. Gaal-Nagy et al., C. R. Physique 10 (2009).

We report a combined experimental and theoretical study of the optical response of epitaxial silicene on silver. The silicene/Ag(111) ultraviolet-visible absorption spectra, which turn out to be strongly nonadditive, are analyzed in the framework of ab initio calculations. Electronic transitions involving silver states are found to provide huge contributions to the optical absorption of silicene, compatible with a strong Si-Ag hybridization. The results are independent of the specific silicene configuration and are also worked out for thin amorphous silicon. This points to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of transient-reflectance spectroscopy. The latter shows a metalliclike relaxation time, hence demonstrating the effects of the strong hybridization arising in silicene/Ag(111) systems on charge carriers dynamical properties.

We report on a theoretical study of the electronic, optical and magneto-optical properties of digital ferromagnetic hetero-structures based on $Mn$ $\delta$--doped $GaAs$. We consider different structures corresponding to $Mn$ contents within the range 12-50\% and we study how the system changes as a function of the doping concentration. Our first--principles approach includes the spin-orbit interaction in a fully relativistic pseudopotential scheme and the local fields effect in the description of the optical absorption. We show that $Mn$ $\delta$-doped $GaAs$ shares many properties with the uniformly doped $Ga_{1-x}Mn_xAs$ system, i.e. half-metallicity, similar absorption spectra, and moderate Kerr rotation angles in the visible spectral region., Comment: in press on Physical Review B

We investigate the long-standing problem of the hole localization at the Al impurity in quartz SiO$_2$, using a relatively recent DFT hybrid-functional method in which the exchange fraction is obtained \emph{ab initio}, based on an analogy with the static many-body COHSEX approximation to the electron self-energy. As the amount of the admixed exact exchange in hybrid functionals has been shown to be determinant for properly capturing the hole localization, this problem constitutes a prototypical benchmark for the accuracy of the method, allowing one to assess to what extent self-interaction effects are avoided. We obtain good results in terms of description of the charge localization and structural distortion around the Al center, improving with respect to the more popular B3LYP hybrid-functional approach. We also discuss the accuracy of computed hyperfine parameters, by comparison with previous calculations based on other self-interaction-free methods, as well as experimental values. We discuss and rationalize the limitations of our approach in computing defect-related excitation energies in low-dielectric-constant insulators., Accepted for publication in J. Chem. Phys. (Communications)

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid density-functional method in which the fraction of exact exchange is material-dependent but obtained ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with the available experimental results, shedding light on the underlying excitation mechanisms and related materials properties. We find that this novel approach is able to reproduce not only ground-state properties and band structures of perfect bulk oxide materials, but also provides results consistent with the optical and electrical behavior observed in the corresponding substoichiometric defective systems., Accepted for publication in J. Chem. Phys

We present an ab initio calculation of the line shape of the photoabsorption spectrum of Na8 and Na+9 clusters at zero temperature, including the polarization effect driven by the electron-plasmon coupling, together with the electron-phonon coupling. The electron-plasmon coupling is found to yield a significant lowering of the energy centroid of the plasmons, whereas the main effect the electron-phonon coupling has is to smooth the spectrum. © 2005 IOP Publishing Ltd.

The electronic and optical properties of silicon nanocrystals passivated with hydrogen and oxygen have been investigated both in the ground- and in an excited-state configuration, through different ab-initio techniques. The presence of an electron–hole pair leads to a strong interplay between the structural and optical properties of the system. The structural distortion of the nanocrystals induced by an electronic excitation is analysed together with the role of the symmetry constraint during the relaxation. The structural distortion can account for the experimentally observed Stokes Shift. Size-related aspects are also analysed and discussed.

A Comment on the Letter by M. Calandra and F. Mauri, Phys. Rev. Lett. 106, 196406 (2011). The authors of the Letter offer a Reply.

We measure direct multiphoton photoemission of the Au(111) surface state with 800-nm laser pulses. We observe the parabolic dispersion in the angular distribution of photoelectrons having absorbed between four and seven photons. The ${\mathbf{k}}_{\ensuremath{\parallel}}$ dispersion we measure can be explained in terms of Shockley-state replicas, with a nascent hot electrons distribution at ${\mathbf{k}}_{\ensuremath{\parallel}}$ above the Fermi level. Moderate laser power densities, of the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{GW}/\mathrm{cm}{}^{2}$, resulted in large electron yields, indicating the importance of multiphoton excitations to define the electronic and magnetic properties of matter in the first hundred femtoseconds after laser excitation.

The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.

We report a combined theoretical and experimental investigation of quantum confinement effects on the E1 interband structures in the optical spectra of Ge quantum dots. The experimental average radius goes from 12 to 60 A. A blueshift of the E1 absorption peak (up to 0.1 eV) and a strong reduction in the oscillator strength, are measured effects of the quantum confinement. The theoretical analysis is performed for nanocrystals up to 15 A through a semi-empirical tight-binding approach. We overcome the single-particle scheme including, in a perturbative approach, the Coulomb electron–hole interaction which induces a red-shift of the E1 peak with respect to the one-electron calculation.

The reliability of the widespread practice of calculating the optical properties of solids using pseudo wavefunctions instead of the true electron wavefunctions has been tested in the case of bulk GaAs. Pseudopotential calculations of the imaginary part of the dielectric function — where the matrix elements of the momentum operator are calculated between pseudo wavefunctions — have been compared with all-electron full-potential linear muffin-tin orbital calculations where the true wavefunctions are used. No evidence has been found of differences due to the different sets of wavefunctions employed in the two approaches.

We have studied the quasiparticle (QP) wave functions and energies of the GaAs(1 1 0) surface and of the cluster Na4 within Hedin's GW approximation. We found that the commonly accepted assumption that the Density Functional Theory (DFT)–Local Density Approximation (LDA) wave functions are a very good approximation for the true QP ones can be wrong. We analyze the effect of the resulting changes in the wave functions on the optical spectra.

We study the two lowest-energy isomers of the Ge(111)-(2 x 1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the silicon case, the stablest isomer differs from the standard "buckled Pandey chains" reconstruction. This conclusion is supported by accurate total-energy results.

The electron--phonon coupling in fullerene C28 has been calculated from first principles. The value of the associated coupling constant lambda/N(0) is found to be a factor three larger than that associated with C60. Assuming similar values of the density of levels at the Fermi surface N(0) and of the Coulomb pseudopotential for C28-based solids as those associated with alkali-doped fullerides A3C60, one obtains Tc(C28) \approx 8 Tc(C60).

The matrix elements of the deformation potential of ${\mathrm{C}}_{70}$ are calculated by means of a simple, yet accurate solution of the electron-phonon coupling problem in fullerenes, based on a parametrization of the ground-state electronic density of the system in terms of ${\mathrm{sp}}^{2+x}$ hybridized orbitals. The value of the calculated dimensionless total electron-phonon coupling constant is $\ensuremath{\lambda}\ensuremath{\approx}0.1,$ an order of magnitude smaller than in ${\mathrm{C}}_{60},$ consistent with the lack of a superconducting phase transition in ${\mathrm{C}}_{70}{\mathrm{A}}_{3}$ fullerite, and in overall agreement with measurements of the broadening of Raman peaks in ${\mathrm{C}}_{70}{\mathrm{K}}_{4}.$ We also calculate the photoemission cross section of ${\mathrm{C}}_{70}^{\ensuremath{-}},$ which is found to display less structure than that associated with ${\mathrm{C}}_{60}^{\ensuremath{-}},$ in overall agreement with the experimental findings.

The generalized Xa method, based on a local, energy-independent operator derived from the exchangecorrelation potential of the local-density approximation by changing the weight of the exchange term, is revisited with the aim of building an optimized starting point for band-structure calculations within the GW method. We find that the optimal choice coincides with the unmodified local-density approximation potential, i.e., with a52/3. Moreover, we show that the use of an Xa method to mimic the self-energy effects in the calculation of absorption spectra leads to worse results than the simpler ‘‘scissors operator’’ approach.

We present ab initio calculations for both the wave functions and the energies of single quasiparticles. The conventional quasiparticle approach evaluates energy-level corrections to first order in the difference between the self-energy and the Kohn-Sham exchange-correlation potential. Here, we also recalculate the quasiparticle states. At the example of the GaAs(110) surface we show that this nonperturbative treatment is important for surfaces with electronic states close in energy but different with respect to their localization. As a sensitive observable the reflectance anisotropy is studied.

local-field effects into account. The results, compared with those obtained in the local-density approximation, show a limited reduction of the well-known overestimation of the macroscopic dielectric constant. Results obtained for optical and energy loss spectra are also presented. @S0163-1829~99!00344-6# I. INTRODUCTION Density-functional theory 1‐3 provides a formally exact tool for the description of all the ground-state properties of many-electron systems at zero temperature, starting from first principles. The only essential approximation introduced in actual calculations is in the exchange-correlation term of the energy functional, since its exact form is unknown. The most widely used expression for the exchange-correlation functional is obtained in the local-density approximation ~LDA!, 3 which, over the past 20 years, has been shown to yield results for crystal structures, lattice constants, elastic constants, and phonon frequencies within a few percent from the experiment, for a wide class of materials. 4,5 A remarkable exception is the static macroscopic dielectric constant « M , whose value is substantially overestimated by density-functional theory ~DFT! LDA. 6‐8 The exact amount of such an overestimate depends on the lattice constant used in the calculation ~and on other factors as pseudopotential types, basis sets, etc. ! but it ranges always between 12% and 18%. This point deserves an accurate discussion. The macroscopic dielectric constant of a system is related to the change in the electronic ground state induced by an external perturbing electrostatic potential dVext . Since, within DFT, the ground-state density r can be computed ~at least in principle! exactly, for both the unperturbed and perturbed systems, the difference dr5(r pert2r 0) can also be obtained exactly. Hence, treating adequately the variation of the external potential ~either by a direct approach or by using perturbation theory!, the dielectric constant should come out correctly within DFT. The large discrepancy with the experiments seems hence to be uniquely due to the LDA. However, it has been recently argued that this is not necessarily true. 9‐11 On the other hand, Levine and Allan 12 performed calculations of « M within a quasiparticle scheme, i.e., in the framework of an excited-state theory. They obtained a value of « M for Silicon within 3% from experiment. However, as pointed out by Dal Corso, Baroni, and Resta, 13 there is no immediate justification for the failure of DFT.

We present a calculation of the electronic and optical properties of the GaAs(110) : H surface performed within the first-principles density functional theory (DFT) in the local density approximation (LDA). The geometry and electronic structure are analyzed and compared with those of the clean surface. The reflectance anisotropy spectrum and the differential reflectivity are then computed. Taking into account the usual underestimation of the LDA gaps with respect to the experimental values, the comparison between our theoretical spectra with available experimental reflectance data shows a satisfactory agreement, allowing for the interpretation of the main structures in terms of electronic transitions.