Real-Time Propagation TDDFT and Density Analysis for Exciton Couplings Calculations in Large Systems

Photo-active systems are characterized by their capacity of absorbing light energy and transforming it. Usually, more than one chromophore is involved in the light absorption and excitation transport processes in complex systems. Linear-Response Time-Dependent Density Functional (LR-TDDFT) is commonly used to identify excitation energies and transition properties by solving well-known Casida's equation for single molecules. However, this methodology is not useful in practice when dealing with multichromophore systems. In this work, we extend our local density decomposition method that enables to disentangle individual contributions into the absorption spectrum to computation of exciton dynamic properties, such as exciton coupling parameters. We derive an analytical expression for the transition density from Real-Time Propagation TDDFT (P-TDDFT) based on Linear Response theorems. We demonstrate the validity of our method to determine transition dipole moments, transition densities and exciton coupling for systems of increasing complexity. We start from the isolated benzaldehyde molecule, perform a distance analysis for $\pi$-stacked dimers and finally map the exciton coupling for a 14 benzaldehyde cluster.Comment: 32 pages, 8 figures; added references in introductions, typos fixe

Nuestro cuerpo también: pensar en precario en la España de la crisis

RESUMENEn este artículo propongo reclamar la existencia de una "constelación ensayística de la vulnerabilidad", analizar las características literario-filosóficas que la definen y reflexionar sobre su relación con el campo intelectual de la España de la última década. Para ello, me acerco en primera instancia a las teorías de la precariedad propuestas por Judith Butler o Isabell Lorey, entre otras, para deslindar conceptos como "vulnerabilidad", "precariedad" y "precarización". Posteriormente esbozo, a modo de muestrario, el corpus ensayístico que configura la constelación mencionada y sugiero una tipología a partir de una serie de categorías propias del sujeto precario. Finalmente selecciono el libro de Raquel Taranilla, Mi cuerpo también, como ejemplo a través del cual distinguir las estrategias discursivas del "pensar en precario" y planteo la posibilidad de intuir, en estas propuestas, una nueva figura del saber que podríamos denominar "el intelectual vulnerable".   ABSTRACTMy aim with this article is to underline the existence of an «essay-like constellation of the vulnerability», analyze the literary-philosophical features that define it and ponder its connection with the Spanish intellectual field of the past decade. With that in mind, I will look, in the first place, towards the theories of precariousness put forward by Judith Butler and Isabell Lorey, among others, to delineate concepts like «vulnerability», «precariousness», and «precarisation». Later, I will outline, as an example, the corpus of essays which give shape to the aforementioned constellation and I propose a tipology based on several of the features of the precarious subject. Finally, I choose to highlight Raquel Taranilla’s book, Mi cuerpo también, as an example to pinpoint the discursive strategies of «precarious thinking» and the possibility of making out, through these approaches, a new figure in the field of knowledge that could be called «the vulnerable intellectual»

Optical properties and exciton transfer between N-heterocyclic carbene iridium (III) complexes for blue light-emitting diode applications from first principles

N-heterocyclic carbene (NHC) iridium (III) complexes are considered as promising candidates for blue emitters in organic light-emitting diodes. They can play the roles of the emitter as well as of electron and hole transporters in the same emission layer. We investigate optical transitions in such complexes with account of geometry and electronic structure changes upon excitation or charging and exciton transfer between the complexes from first principles. It is shown that excitation of NHC iridium complexes is accompanied by a large reorganization energy $\sim$0.7 eV and a significant loss in the oscillator strength, which should lead to low exciton diffusion. Calculations with account of spin-orbit coupling reveal a small singlet-triplet splitting $\sim$0.1 eV, whereas the oscillator strength for triplet excitations is found to be an order of magnitude smaller than for the singlet ones. The contributions of the F\"orster and Dexter mechanisms are analyzed via the explicit integration of transition densities. It is shown that for typical distances between emitter complexes in the emission layer, the contribution of the Dexter mechanism should be negligible compared to the F\"orster mechanism. On the other hand, the ideal dipole approximation, although gives the correct order of the exciton coupling, fails to reproduce the result taking into account spatial distribution of the transition density. For charged NHC complexes, we find a number of optical transitions close to the emission peak of the blue emitter with high exciton transfer rates that can be responsible for exciton-polaron quenching. The nature of these transitions is analyzed.Comment: 20 pages, 7 figure

Crítica de la razón precaria. La vida intelectual ante la obligación de lo extraordinario

Review of LÓPEZ ALÓS, Javier (2019). Crítica de la razón precaria. La vidaintelectual ante la obligación de lo extraordinario. Madrid: Catarata, pp. 140.Reseña de: LÓPEZ ALÓS, Javier (2019). Crítica de la razón precaria. La vida intelectual ante la obligación de lo extraordinario. Madrid: Catarata, pp. 140

A definition of the magnetic transition temperature using valence bond theory

Macroscopic magnetic properties are analyzed using Valence Bond theory. Commonly the critical temperature TC for magnetic systems is associated with a maximum in the energy-based heat capacity Cp(T). Here a more broadly applicable definition of the magnetic transition temperature TC is described using spin moment expectation value (i.e. applying the spin exchange density operator) instead of energy. Namely, the magnetic capacity Cs(T) reflects variation in the spin multiplicity as a function of temperature, which is shown to be related to ∂[χT(T)]/∂T. Magnetic capacity Cs(T) depends on long-range spin interactions that are not relevant in the energy-based heat capacity Cp(T). Differences between Cs(T) and Cp(T) are shown to be due to spin order/disorder within the crystal, that can be monitored via a Valence Bond analysis of the corresponding magnetic wavefunction. Indeed the concept of the Boltzmann spin-alignment order is used to provide information about the spin correlation between magnetic units. As a final illustration, the critical temperature is derived from the magnetic capacity for several molecular magnets presenting different magnetic topolo- gies that have been experimentally studied. A systematic shift between the transition temperatures associated with Cs(T) and Cp(T) is observed. It is demonstrated that this shift can be attributed to the loss of long-range spin correlation. This suggests that the magnetic capacity Cs(T) can be used as a predictive tool for the magnetic topology, and thus for the synthetic chemists

Error estimation in IBM Quantum Computers

One of the main important features of the noisy intermediate-scale quantum (NISQ) era is the correct evaluation and consideration of errors. In this paper, we analyze the main sources of errors in current (IBM) quantum computers and we present a useful tool (TED-qc) designed to facilitate the total error probability expected for any quantum circuit. We propose this total error probability as the best way to estimate the fidelity in the NISQ era, mainly because we do not have to compare our quantum calculations with any classical one. In order to contrast the robustness of our tool we compute the total error probability that may occur in three different quantum models: 1) the four-electron Ising model, 2) the Quantum-Phase Estimation (QPE) and 3) the Grover's algorithm. For each model, we compute a statistically significant sample size for both the expectation value of the related observable and the fidelity, comparing them with the value calculated in the simulator as a function of the error probability. The analysis is satisfactory in more than the $99\%$ of the cases. In addition, we study how the error mitigation techniques are able to eliminate the noise induced during the measurement.Comment: 9 pages, 8 figure