Many-body Green's function GW and Bethe-Salpeter study of the optical excitations in a paradigmatic model dipeptide

We study within the many-body Green's function GW and Bethe-Salpeter formalisms the excitation energies of a paradigmatic model dipeptide, focusing on the four lowest-lying local and charge-transfer excitations. Our GW calculations are performed at the self-consistent level, updating first the quasiparticle energies, and further the single-particle wavefunctions within the static Coulomb-hole plus screened-exchange approximation to the GW self-energy operator. Important level crossings, as compared to the starting Kohn-Sham LDA spectrum, are identified. Our final Bethe-Salpeter singlet excitation energies are found to agree, within 0.07 eV, with CASPT2 reference data, except for one charge-transfer state where the discrepancy can be as large as 0.5 eV. Our results agree best with LC-BLYP and CAM-B3LYP calculations with enhanced long-range exchange, with a 0.1 eV mean absolute error. This has been achieved employing a parameter-free formalism applicable to metallic or insulating extended or finite systems.Comment: 25 pages, 5 figure

Electrons, excitons et polarons dans les systèmes organiques : approches ab initio à N-corps de type GW et Bethe-Salpeter pour le photovoltaïque organique

The present thesis aims at exploring the properties and merits of the ab initio Green's function many-body perturbation theory (MBPT) GW and Bethe-Salpeter formalisms, in order to provide a well-grounded and accurate description of the electronic and optical properties of condensed matter systems. While these approaches have been developed for extended inorganic semiconductors and extensively tested on this class of systems since the 60 s, the present work wants to assess their quality for gas phase organic molecules, where systematic studies still remain scarce. By means of small isolated study case molecules, we want to progress in the development of a theoretical framework, allowing an accurate description of complex organic systems of interest for organic photovoltaic devices. This represents the main motivation of this scientific project and we profit here from the wealth of experimental or high-level quantum chemistry reference data, which is available for these small, but paradigmatic study cases.This doctoral thesis came along with the development of a specific tool, the FIESTA package, which is a Gaussian basis implementation of the GW and Bethe-Salpeter formalisms applying resolution of the identity techniques with auxiliary bases and a contour deformation approach to dynamical correlations. Initially conceived as a serial GW code, with limited basis sets and functionalities, the code is now massively parallel and includes the Bethe-Salpeter formalism. The capacity to perform calculations on several hundreds of atoms to moderate costs clearly paves the way to enlarge our studies from simple model molecules to more realistic organic systems. An ongoing project related to the development of discrete polarizable models accounting for the molecular environment allowed me further to become more familiar with the actual implementation and code structure.The manuscript at hand is organized as follows. In an introductory chapter, we briefly present the basic mechanisms characterizing organic solar cells, accentuating the properties which seek for an accurate theoretical description in order to provide some insight into the factors determining solar cell efficiencies. The first chapter of the main part is methodological, including a discussion of the principle features and approximations behind standard mean-field techniques (Hartree, Hartree-Fock, density functional theory). Starting from a description of photoemission experiments, the MBPT and quasiparticle ideas are introduced, leading to the so-called Hedin's equations, the GW method and the COHSEX approach. In order to properly describe optical experiments, electron-hole interactions are included on top of the description of inter-electronic correlations. In this context, the Bethe-Salpeter formalism is introduced, along with an excursus on time-dependent density functional theory. Chapter 2 briefly presents the technical specifications of the GW and Bethe-Salpeter implementation in the FIESTA package. The properties of Gaussian basis sets, the ideas behind the resolution of the identity techniques and finally the contour deformation approach to dynamical correlations are discussed. The third chapter deals with the results obtained during this doctoral thesis. On the electronic structure level, a recent study on a paradigmatic dipeptide molecule will be presented. Further, also its optical properties will be explored, together with an in-depth discussion of charge-transfer excitations in a family of coumarin molecules. Finally, by means of the Buckminster fullerene C60 and the two-dimensional semi-metal graphene, we will analyze the reliability of two many-body formalisms, the so-called static COHSEX and constant-screening approximation, for an efficient calculation of electron-phonon interactions in organic systems at the MBPT level. After a short conclusion, the Appendix containing details and derivations of the formalisms presented before closes this work.Cette thèse se propose d'explorer les mérites d'une famille d'approches de simulation quantique ab initio, les théories de perturbation à N-corps, pour l'exploration des propriétés électroniques et optiques de systèmes organiques. Nous avons étudié en particulier l'approximation dite de GW et l'équation de Bethe-Salpeter, très largement utilisées dès les années soixante pour les semiconducteurs de volume, mais dont l'utilisation pour les systèmes organiques moléculaires est très limitée. L'étude de quelques cas d'intérêt pour le photovoltaïque organique, et en particulier de petites molécules pour lesquelles sont disponibles des données expérimentales ou des résultats issus d'approches de chimie quantique, nous ont permis de valider ces approches issues de la physique du solide.Ce doctorat s'inscrit dans le cadre du développement d'un outil de simulation quantique spécifique (le projet FIESTA) dont l'objectif est de combiner les formalismes GW et Bethe-Salpeter avec les techniques de la chimie quantique, c'est-à-dire en particulier l'utilisation de bases localisées analytiques (bases gaussiennes) et des approches de type «résolution de l'identité» pour le traitement des intégrales Coulombiennes. Ce code est aujourd'hui massivement parallélisé, permettant, au delà des études de validation présentées dans ce travail de thèse, l'étude de systèmes complexes comprenant plusieurs centaines d'atomes. En cours de développement, l'incorporation d'approches hybrides combinant mécanique quantique et écrantage à longue portée par des approches modèles de milieu polarisable m'a permis d'une part de me familiariser avec le code et le développement méthodologique, et permet d'autre part d'envisager l'étude de systèmes réalistes en couplage avec leur environnement.Le manuscrit s‘ouvre sur une introduction au photovoltaïque organique afin de mettre en lumière les questionnements spécifiques qui requièrent le développement de nouveaux outils théoriques à la fois fiables en terme de précision et suffisamment efficaces pour traiter des systèmes de grande taille. Le premier chapitre est d'ordre méthodologique et rappelle les fondements des techniques ab initio de type champ-moyen (Hartree, Hartree-Fock et théorie de la fonctionnelle de la densité). En partant des principes de la photoémission, les théories de perturbation à N-corps et la notion de quasi-particule sont ensuite introduites, conduisant aux équations de Hedin et aux approximations GW et COHSEX. De même, à partir de la compréhension d'une expérience d'optique, le traitement des interactions électron-trou est présenté, menant à l'équation de Bethe-Salpeter. Le chapitre 2 introduit brièvement les spécificités techniques liées à l'implémentation des formalismes GW et Bethe-Salpeter. Les propriétés analytiques des bases gaussiennes et les principes mathématiques derrière les techniques de type «résolution de l'identité» et «déformation de contour», sont brièvement décrites. Le troisième chapitre présente les résultats scientifiques obtenus durant cette thèse. Le cas paradigmatique d'un polypeptide model nous permettra de discuter des spécificités de l'approche GW appliquée à des systèmes moléculaires afin d'obtenir des énergies de quasiparticule de bonne qualité. De même, l'utilisation de l'équation de Bethe-Salpeter pour l'obtention du spectre optique de ce système sera présentée, ainsi que le cas d'une famille de colorants d'importance pour les cellules de Graetzel (les coumarines). Finalement, nous explorons dans le cas du fullerène C60 et du graphène le calcul des termes de couplage électron-phonon dans le cadre de l'approche GW, c'est-à-dire au delà des approches standards de type théorie de la fonctionnelle de la densité. Notre étude vise à vérifier si une approximation statique et à écrantage constant au premier ordre permet de garder la qualité des résultats GW pour un coût numérique réduit. Après la conclusion, les appendices donnent le détail de certaines dérivations

First-principles GW calculations for DNA and RNA nucleobases

On the basis of first-principles GW calculations, we study the quasiparticle properties of the guanine, adenine, cytosine, thymine, and uracil DNA and RNA nucleobases. Beyond standard G0W0 calculations, starting from Kohn-Sham eigenstates obtained with (semi)local functionals, a simple self-consistency on the eigenvalues allows to obtain vertical ionization energies and electron affinities within an average 0.11 eV and 0.18 eV error respectively as compared to state-of-the-art coupled-cluster and multi-configurational perturbative quantum chemistry approaches. Further, GW calculations predict the correct \pi -character of the highest occupied state, thanks to several level crossings between density functional and GW calculations. Our study is based on a recent gaussian-basis implementation of GW with explicit treatment of dynamical screening through contour deformation techniques.Comment: 5 pages, 3 figure

Electron-phonon coupling in the C60 fullerene within the many-body GW approach

We study the electron-phonon coupling in the C60 fullerene within the first-principles GW approach, focusing on the lowest unoccupied t1u three-fold electronic state which is relevant for the superconducting transition in electron doped fullerides. It is shown that the strength of the coupling is significantly enhanced as compared to standard density functional theory calculations with (semi)local functionals, with a 48% increase of the electron-phonon potential Vep. The calculated GW value for the contribution from the Hg modes of 93 meV comes within 4% of the most recent experimental values. The present results call for a reinvestigation of previous density functional based calculations of electron-phonon coupling in covalent systems in general.Comment: 4 pages, 0 figur

Electron-phonon coupling and charge-transfer excitations in organic systems from many-body perturbation theory

International audienceWe review in this article recent developments within the framework of ab initio many-body perturbation theory aiming at providing an accurate description of the electronic and excitonic properties of π-conjugated organic systems currently used in organic photovoltaic cells. In particular, techniques such as the GW and Bethe-Salpeter formalisms are being benchmarked for acenes, fullerenes, porphyrins, phthalocyanines, and other molecules of interest for solar energy applications. It is shown that not only the electronic properties, but also the electron-phonon coupling matrix elements, and the charge-transfer excitations in donor/acceptor complexes, are accurately described. The present calculations on molecules containing up to a hundred atoms are based on a recently developed Gaussian auxiliary basis implementation of the GW and Bethe-Salpeter formalism, including full dynamics with contour-deformation techniques, as implemented in the Fiesta code

Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO₃ perovskites

We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity, and magnetism in the perovskite manganite series, AMnO3 (A = Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic (FM) over antiferromagnetic (AFM) ordering due to suppressed AFM superexchange as the polar distortion bends the Mn–O–Mn bond angles away from the optimal 180°. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for FM than AFM orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure, and magnetism can be modified using chemical potential, stress or pressure, and electric or magnetic fields

Adequação calórico-proteica de pacientes internados em uma unidade de terapia intensiva

Background and Objectives: Enteral Nutritional Therapy (NER) is used for the maintenance or recovery of nutritional status in individuals with functioning gastrointestinal tract, with oral intake partially or totally compromised. The objective of this study was to evaluate the adequacy of NER between the prescribed and administered in patients hospitalized in an Intensive Care Unit of a Teaching Hospital in the interior of Rio Grande do Sul. Methods: This is a descriptive, cross-sectional and quantitative study of 25 patients aged ≥18 years, who were on exclusive NSTEM for a period ≥72 hours. Data collection was performed between September and December 2017 through a Nutrition Service form and the follow-up was carried out until the discharge of the unit or death or discontinuation of the exclusive TNE. Appropriate energy and protein administration ≥ 80% of the needs were considered. The data were analyzed through descriptive statistics and tested by Spearman Correlation in the SPSS program version 20.0. Results: Of the patients evaluated, the majority were overweight (52%), pulmonary disease (28%), started with early nutrition in up to 48 hours (92%), were discharged from home (68%) and 20.0% or diarrhea. We observed that the adequacy of the caloric value was 88.25% ± 2.55, similar to the protein adequacy 88.49% ± 2.51. Conclusion: There was an adequacy between the calories and the prescribed and administered protein higher than expected, allowing an effective caloric / protein administration, thus being more efficient the nutritional conduct.Justificación y objetivos: La Terapia Nutricional Enteral (TNE) es utilizada para el mantenimiento o recuperación del estado nutricional en los individuos con el tracto gastrointestinal funcional, con la ingestión oral parcial o totalmente comprometida. El objetivo de este estudio fue evaluar la adecuación de la TNE entre lo prescrito y administrado en pacientes internados en una Unidad de Terapia Intensiva de un Hospital de Enseñanza del interior del Río Grande del Sur. Métodos: Se trata de un estudio descriptivo, transversal y cuantitativo realizado con 25 pacientes con edad ≥18 años, que estaban en TNE exclusiva por período ≥72 horas. La recolección de datos fue realizada entre septiembre a diciembre de 2017 por medio de un formulario del Servicio de Nutrición y el seguimiento fue realizado hasta el alta de la unidad o muerte o discontinuación de la TNE exclusiva. Se consideró, como adecuada la administración energética y protege el 80% de las necesidades. Los datos fueron analizados a través de estadística descriptiva y probado a Correlación de Spearman en el programa SPSS versión 20.0. Resultados: De los pacientes evaluados, la mayoría presentaba sobrepeso (52%), enfermedad pulmonar (28%), iniciaron con la nutrición precoz en hasta 48 horas (92%), tuvieron alta domiciliar (68%) y 20,0% presentaron vómito o diarrea. Se observó que la adecuación del valor calórico quedó 88,25% ± 2,55, similar a la adecuación de proteína 88,49% ± 2,51. Conclusiones: Hubo una adecuación entre las calorías y la proteína prescrita y administrada superior a lo esperado, permitiendo una efectiva administración calórico / proteica, siendo así más eficaz la conducta nutricional.RESUMO Justificativa e Objetivo: A Terapia Nutricional Enteral (TNE) é utilizada para a manutenção ou recuperação do estado nutricional nos indivíduos com o trato gastrintestinal funcionante, com a ingestão oral parcial ou totalmente comprometida. O objetivo deste estudo foi avaliar o uso de TNE exclusiva, em relação ao que é prescrito e ao que é administrado em pacientes internados em uma Unidade de Terapia Intensiva (UTI) de um Hospital de Ensino do interior do Rio Grande do Sul. Métodos: Trata-se de um estudo de coorte, prospectivo, em que foram incluídos 25 pacientes com idade ≥18 anos, que estavam em TNE exclusiva por período ≥72 horas. A coleta de dados foi realizada entre setembro a dezembro de 2017. Os dados coletados foram registrados no formulário do Serviço de Nutrição e o acompanhamento foi realizado até a alta da unidade ou óbito ou descontinuação da TNE exclusiva. Considerou-se, como adequada a administração energética e proteica ≥ 80% das necessidades. Utilizou-se o SPSS versão 20.0 para analisar as medidas descritivas e foi testada a Correlação de Spearman. Resultados: Dos pacientes avaliados, 52,0% apresentavam sobrepeso, 28,0% apresentavam doença pulmonar, 92,0% iniciaram com a nutrição precoce em até 48 horas, 68,0% tiveram alta domiciliar e 20,0% apresentaram vômito ou diarreia. Observamos que a adequação do valor calórico ficou 88,25% ± 2,55, semelhante a adequação de proteína 88,49% ± 2,51. Conclusão: A maioria dos pacientes receberam fórmula hipercalórica, com uma adequação entre as calorias e a proteína prescrita e administrada superior ao esperado pelo presente estudo

Adequação calórico-proteica de pacientes internados em uma unidade de terapia intensiva

RESUMO Justificativa e Objetivo: A Terapia Nutricional Enteral (TNE) é utilizada para a manutenção ou recuperação do estado nutricional nos indivíduos com o trato gastrintestinal funcionante, com a ingestão oral parcial ou totalmente comprometida. O objetivo deste estudo foi avaliar o uso de TNE exclusiva, em relação ao que é prescrito e ao que é administrado em pacientes internados em uma Unidade de Terapia Intensiva (UTI) de um Hospital de Ensino do interior do Rio Grande do Sul. Métodos: Trata-se de um estudo de coorte, prospectivo, em que foram incluídos 25 pacientes com idade ≥18 anos, que estavam em TNE exclusiva por período ≥72 horas. A coleta de dados foi realizada entre setembro a dezembro de 2017. Os dados coletados foram registrados no formulário do Serviço de Nutrição e o acompanhamento foi realizado até a alta da unidade ou óbito ou descontinuação da TNE exclusiva. Considerou-se, como adequada a administração energética e proteica ≥ 80% das necessidades. Utilizou-se o SPSS versão 20.0 para analisar as medidas descritivas e foi testada a Correlação de Spearman. Resultados: Dos pacientes avaliados, 52,0% apresentavam sobrepeso, 28,0% apresentavam doença pulmonar, 92,0% iniciaram com a nutrição precoce em até 48 horas, 68,0% tiveram alta domiciliar e 20,0% apresentaram vômito ou diarreia. Observamos que a adequação do valor calórico ficou 88,25% ± 2,55, semelhante a adequação de proteína 88,49% ± 2,51. Conclusão: A maioria dos pacientes receberam fórmula hipercalórica, com uma adequação entre as calorias e a proteína prescrita e administrada superior ao esperado pelo presente estudo

ARTEFACTS: How do we want to deal with the future of our one and only planet?

The European Commission’s Science and Knowledge Service, the Joint Research Centre (JRC), decided to try working hand-in-hand with leading European science centres and museums. Behind this decision was the idea that the JRC could better support EU Institutions in engaging with the European public. The fact that European Union policies are firmly based on scientific evidence is a strong message which the JRC is uniquely able to illustrate. Such a collaboration would not only provide a platform to explain the benefits of EU policies to our daily lives but also provide an opportunity for European citizens to engage by taking a more active part in the EU policy making process for the future. A PILOT PROGRAMME To test the idea, the JRC launched an experimental programme to work with science museums: a perfect partner for three compelling reasons. Firstly, they attract a large and growing number of visitors. Leading science museums in Europe have typically 500 000 visitors per year. Furthermore, they are based in large European cities and attract local visitors as well as tourists from across Europe and beyond. The second reason for working with museums is that they have mastered the art of how to communicate key elements of sophisticated arguments across to the public and making complex topics of public interest readily accessible. That is a high-value added skill and a crucial part of the valorisation of public-funded research, never to be underestimated. Finally museums are, at present, undergoing something of a renaissance. Museums today are vibrant environments offering new techniques and technologies to both inform and entertain, and attract visitors of all demographics.JRC.H.2-Knowledge Management Methodologies, Communities and Disseminatio