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Theoretical description of photoabsorption spectra: TDDFT development and applications
No full textLo scopo principale di questa tesi è la simulazione teorica e quindi l’interpretazione di spettri di fotoassorbimento. Questo progetto può essere suddiviso in due argomenti: il calcolo di spettri Near Edge X-ray Absorption Fine Structure (NEXAFS) a livello DFT per molecole in fase gassosa e lo sviluppo di un nuovo algoritmo per il calcolo di spettri di fotoassorbimento a livello TDDFT per sistemi di grandi dimensioni (>100 atomi).
Gli spettri NEXAFS C1s e XPS in fase gassosa di due idrocarburi policiclici aromatici (fenantrene e coronene) sono stati calcolati, interpretati e confrontati con i dati sperimentali. Gli spettri calcolati a livello ΔKS considerando solo le transizioni elettroniche presentano delle differenze significative rispetto all’esperimento, in particolare per quanto riguarda la distribuzione delle intensità. L’inclusione dell’accoppiamento vibronico a livello Frank-Condon nel modello di calcolo migliora fortemente l’accordo della distribuzione dell’intensità rispetto all’esperimento. Inoltre sono stati calcolati e interpretati gli spettri NEXAFS in fase gassosa a livello ΔKS per una serie di tiofeni (tiofene, ditiofene e tertiofene), i dati teorici sono in buon accordo con i dati sperimentali.
La seconda parte di questa tesi verte sullo sviluppo di un nuovo algoritmo TDDFT per il calcolo di spettri di fotoassorbimento che superi i limiti intrinseci degli attuali codici che utilizzano l’algoritmo di Casida e quindi la diagonalizzazione di Davidson. Infatti quando si studiano sistemi di grandi dimensioni (>100 atomi) come ad esempio cluster metallici la densità degli stati è molto elevata e di conseguenza con algoritmi che utilizzino la diagonalizzazione di Davidson è possibile il calcolo di una regione limitata di spettro, il nuovo algoritmo risolve le equazioni TDDFT nello spazio del set di base ausiliaria delle funzioni di fitting della densità, questo metodo estrae lo spettro punto per punto dalla parte immaginaria della polarizzabilità. L’idea principale che rende praticabile questo metodo è la semplificazione della doppia somma occupato-virtuale per il calcolo della suscettibilità dielettrica, rendendo il calcolo di questa matrice una combinazione lineare tra matrici costanti e coefficienti dipendenti dall’energia. Questo metodo è stato applicato per lo studio di diversi sistemi per natura e dimensioni (da H2 a [Au147]-) e presenta un buon accordo con calcoli effettuati utilizzando il codice ADF (massima differenza di 0.2eV). Inoltre è stata implementata la possibilità di analisi dei risultati tramite la Transition Contribution Maps, il fattore di scala di Jacob e l’analisi della densità indotta.
Il nuovo algoritmo è stato infine applicato allo studio di diversi sistemi di cluster metallici, simulando i fenomeni di risonanza plasmonica e gli andamenti rispetto alle dimensioni e alla natura dei leganti. In particolare è stata studiata una serie di icosaedri di argento e oro (M13, M55, M147, M309), una serie di cluster di Au30 con diversi leganti tiolici (terbutiltiolo, feniltiolo, nitrofeniltiolo), inoltre è stato possibile studiare sistemi di grandi dimensioni e a bassa simmetria come Au144(SH)60
The near-edge X-ray-absorption fine-structure of O2 chemisorbed on Ag(110) surface studied by density functional theory
No full textIn this computational work a finite cluster model approach has been employed to simulate the adsorption of
O2 molecule on the Ag(110) surface by means of density functional theory. O 1s NEXAFS spectra have been
calculated with the transition potential scheme in order to include the core hole relaxation effect. From the
analysis of the density of states, experimental and calculated NEXAFS spectra found that the O2 molecule
upon adsorption on Ag(110) has completely filled π* orbitals consistent with a closed shell electronic structure
of peroxide. The only virtual valence molecular orbital still belonging to the O2 fragment is the antibonding
σ*, which will be responsible for the most intense resonance observed and calculated in the O 1s NEXAFS
spectra. The simulated polarized spectra are in good agreement with two different sets of experimental data,
with the exception of grazing incidence, for which a disagreement between theory and experiment is found.
We attributed this disagreement to a slight deviation of the adsorption geometry with respect to the ideal
case, suggesting a tilt angle of about 10°–15° of the O\O bond with respect to the surface plane
A new time dependent density functional algorithm for large systems and plasmons in metal clusters
Full text linkA 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 implemente
Pd doping, conformational, and charge effects on the dichroic response of a monolayer protected Au38(SR)24 nanocluster
No full textTDDFT 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 new time-dependent density-functional method for molecular plasmonics: Formalism, implementation, and the Au144(SH)60 case study
Full text linkWe 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
Extension of the Time-Dependent Density Functional Complex Polarizability Algorithm to Circular Dichroism: Implementation and Applications to Ag8 and Au38(SC2H4C6H5)24
No full textWe 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 Ag8 (as a validation case), Au38(SCH3)24 and Au38(SCH2CH2Ph)24 monolayer-protected gold clusters (as a system of great current interest for their optical properties). For Au38(SCH2CH2Ph)24, the computed CD spectrum agree 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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