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There is a pressing need to develop new rechargeable battery technologies that can offer higher energy storage, faster charging, and lower costs. Despite the success of existing methods for the simulation of battery materials, they can sometimes fall short of delivering accurate and reliable results. Quantum computing has been discussed as an avenue to overcome these issues, but only limited work has been done to outline how they may impact battery simulations. In this work, we provide a detailed answer to the following question: how can a quantum computer be used to simulate key properties of a lithium-ion battery? Based on recently-introduced first-quantization techniques, we lay out an end-to-end quantum algorithm for calculating equilibrium cell voltages, ionic mobility, and thermal stability. These can be obtained from ground-state energies of materials, which is the core calculation executed by the quantum computer using qubitization-based quantum phase estimation. The algorithm includes explicit methods for preparing approximate ground states of periodic materials in first quantization. We bring these insights together to perform the first estimation of the resources required to implement a quantum algorithm for simulating a realistic cathode material, dilithium iron silicate., 31 pages, 14 figures

Classical algorithms for predicting the equilibrium geometry of strongly correlated molecules require expensive wave function methods that become impractical already for few-atom systems. In this work, we introduce a variational quantum algorithm for finding the most stable structure of a molecule by explicitly considering the parametric dependence of the electronic Hamiltonian on the nuclear coordinates. The equilibrium geometry of the molecule is obtained by minimizing a more general cost function that depends on both the quantum circuit and the Hamiltonian parameters, which are simultaneously optimized at each step. The algorithm is applied to find the equilibrium geometries of the $\mathrm{H}_2$, $\mathrm{H}_3^+$, $\mathrm{BeH}_2$ and $\mathrm{H}_2\mathrm{O}$ molecules. The quantum circuits used to prepare the electronic ground state for each molecule were designed using an adaptive algorithm where excitation gates in the form of Givens rotations are selected according to the norm of their gradient. All quantum simulations are performed using the PennyLane library for quantum differentiable programming. The optimized geometrical parameters for the simulated molecules show an excellent agreement with their counterparts computed using classical quantum chemistry methods., 7 pages, 4 figures, 1 codeblock

We develop an extension of the time-dependent equation-of-motion formulation of the polarizable continuum model (EOM-TDPCM) to introduce nonequilibrium cavity field effects in quantum mechanical calculations of solvated molecules subject to time-dependent electric fields. This method has been implemented in Octopus, a state-of-the-art code for real-space, real-time time-dependent density functional theory (RT-TDDFT) calculations. To show the potential of our methodology, we perform EOM-TDPCM/RT-TDDFT calculations of trans-azobenzene in water and in other model solvents with shorter relaxation times. Our results for the optical absorption spectrum of trans-azobenzene show (i) that cavity field effects have a clear impact in the overall spectral shape and (ii) that an accurate description of the solute shape (as the one provided within PCM) is key to correctly account for cavity field effects., published under ACS Authorchoice license

An accurate description of electron transport at a molecular level requires a precise treatment of quantum effects. These effects play a crucial role in determining the electron transport properties of single molecules, such as current-voltage curves, which can be challenging to simulate classically. Here we introduce a quantum algorithm to efficiently calculate the electronic current through single-molecule junctions in the weak-coupling regime. We show that a quantum computer programmed to simulate vibronic transitions between different charge states of a molecule can be used to compute sequential electron transfer rates and electric current. In the harmonic approximation, the algorithm can be implemented using Gaussian boson sampling devices, which are a near-term platform for photonic quantum computing. We apply the algorithm to simulate the current and conductance of a magnesium porphine molecule. The simulations demonstrate quantum effects that are manifested as discrete steps in the current and conductance, in agreement with experimental and theoretical data.

Universal gate sets for quantum computing have been known for decades, yet no universal gate set has been proposed for particle-conserving unitaries, which are the operations of interest in quantum chemistry. In this work, we show that controlled single-excitation gates in the form of Givens rotations are universal for particle-conserving unitaries. Single-excitation gates describe an arbitrary $U(2)$ rotation on the two-qubit subspace spanned by the states $|01\rangle, |10\rangle$, while leaving other states unchanged -- a transformation that is analogous to a single-qubit rotation on a dual-rail qubit. The proof is constructive, so our result also provides an explicit method for compiling arbitrary particle-conserving unitaries. Additionally, we describe a method for using controlled single-excitation gates to prepare an arbitrary state of a fixed number of particles. We derive analytical gradient formulas for Givens rotations as well as decompositions into single-qubit and CNOT gates. Our results offer a unifying framework for quantum computational chemistry where every algorithm is a unique recipe built from the same universal ingredients: Givens rotations., Comment: 11 pages, 12 figures

Patient: Male, 38-year-old Final Diagnosis: Recurrent idiopathic acute pancreatitis with COVID-19 Symptoms: Nausea • severe abdominal pain • fever • vomiting Medication:— Clinical Procedure: — Specialty: Gastroenterology and Hepatology • Infectious Diseases • General and Internal Medicine Objective: Unusual clinical course Background: The novel COVID-19 disease has infected more than 2 million people worldwide, causing more than 120 000 deaths. While the disease is known to primarily affect the respiratory system, gastrointestinal manifestations can also occur. However, little is known about the development of acute pancreatitis in COVID-19. The present report highlights a patient with no precipitating risk factors for pancreatitis who presented with recurring acute pancreatitis following the diagnosis of SARS-CoV-2 infection. Case Report: An otherwise healthy 38-year-old man presented to the Emergency Department (ED) with fever and epigastric pain. Laboratory testing revealed a lipase level of 10 255 ukat/L. An abdominal ultrasound showed no gallstones. After ruling out the possible causes of acute pancreatitis, a diagnosis of idiopathic acute pancreatitis was made. He received conservative management and was discharged home after being medically stabilized. Of note, the patient tested positive for SARS-CoV-2 infection at a local testing center 1 week prior to presenting to the ED. One week following the discharge, the patient returned with recurrent severe epigastric pain. Laboratory testing showed a lipase level of 20 320 ukat/L. An abdominal CT revealed acute pancreatitis. Further workups, including abdominal ultrasound, hepatitis serology, and immunoglobulin G for autoimmune pancreatitis, were unrevealing. Repeated SARS-CoV-2 testing produced positive results. Conclusions: The temporal relationship between clinical presentation of acute pancreatitis and SARS-CoV-2 infection in this patient with no precipitating risk factors for pancreatitis suggests COVID-19-associated acute pancreatitis. Our review of the literature found a handful of reported cases of acute pancreatitis in patients with coexisting SARS-CoV-2 infection, and this report presents the first presumptive case of COVID-19-associated recurring acute pancreatitis.

The excitation of vibrational modes in molecules affects the outcome of chemical reactions, for example by providing molecules with sufficient energy to overcome activation barriers. In this work, we introduce a quantum algorithm for simulating molecular vibrational excitations during vibronic transitions. We discuss how a special-purpose quantum computer can be programmed with molecular data to optimize a vibronic process such that desired modes get excited during the transition. We investigate the effect of such excitations on selective bond dissociation in pyrrole and butane during photochemical and mechanochemical vibronic transitions. The results are discussed with respect to experimental observations and classical simulations. We also introduce quantum-inspired classical algorithms for simulating molecular vibrational excitations in special cases where only a limited number of modes are of interest., 10 pages, 9 figures

Over the last few years, extraordinary advances in experimental and theoretical tools have allowed us to monitor and control matter at short time and atomic scales with a high degree of precision. An appealing and challenging route toward engineering materials with tailored properties is to find ways to design or selectively manipulate materials, especially at the quantum level. To this end, having a state-of-the-art ab initio computer simulation tool that enables a reliable and accurate simulation of light-induced changes in the physical and chemical properties of complex systems is of utmost importance. The first principles real-space-based Octopus project was born with that idea in mind, i.e., to provide a unique framework that allows us to describe non-equilibrium phenomena in molecular complexes, low dimensional materials, and extended systems by accounting for electronic, ionic, and photon quantum mechanical effects within a generalized time-dependent density functional theory. This article aims to present the new features that have been implemented over the last few years, including technical developments related to performance and massive parallelism. We also describe the major theoretical developments to address ultrafast light-driven processes, such as the new theoretical framework of quantum electrodynamics density-functional formalism for the description of novel light–matter hybrid states. Those advances, and others being released soon as part of the Octopus package, will allow the scientific community to simulate and characterize spatial and time-resolved spectroscopies, ultrafast phenomena in molecules and materials, and new emergent states of matter (quantum electrodynamical-materials)., This work was supported by the European Research Council (Grant No. ERC-2015-AdG694097), the Cluster of Excellence “Advanced Imaging of Matter” (AIM), Grupos Consolidados (IT1249-19), and SFB925. The Flatiron Institute is a division of the Simons Foundation. X.A., A.W., and A.C. acknowledge that part of this work was performed under the auspices of the U.S. Department of Energy at Lawrence Livermore National Laboratory under Contract No. DE-AC52-07A27344. J.J.-S. gratefully acknowledges the funding from the European Union Horizon 2020 Research and Innovation Program under the Marie Sklodowska-Curie Grant Agreement No. 795246-StrongLights. J.F. acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG Forschungsstipendium FL 997/1-1). D.A.S. acknowledges University of California, Merced start-up funding.

Gaussian Boson Sampling (GBS) is a near-term platform for photonic quantum computing. Recent efforts have led to the discovery of GBS algorithms with applications to graph-based problems, point processes, and molecular vibronic spectra in chemistry. The development of dedicated quantum software is a key enabler in permitting users to program devices and implement algorithms. In this work, we introduce a new applications layer for the Strawberry Fields photonic quantum computing library. The applications layer provides users with the necessary tools to design and implement algorithms using GBS with only a few lines of code. This paper serves a dual role as an introduction to the software, supported with example code, and also a review of the current state of the art in GBS algorithms., Code available at https://github.com/XanaduAI/strawberryfields/ and documentation available at https://strawberryfields.readthedocs.io/

We determine here the evolution of the bandgap energy with size in graphene quantum dots (GQDs). We find oscillatory behaviour of the bandgap and explain its origin in terms of armchair and zigzag edges. The electronic energy spectra of GQDs are computed using both the tight binding model and {\it ab-initio} density functional methods. The results of the tight binding model are analyzed by dividing zigzag graphene quantum dots into concentric rings. For each ring, the energy spectra, the wave functions and the bandgap are obtained analytically. The effect of inter-ring tunneling on the energy gap is determined. The growth of zigzag terminated GQD into armchair GQD is shown to be associated with the addition of a one-dimensional Lieb lattice of carbon atoms with a shell of energy levels in the middle of the energy gap of the inner zigzag terminated GQD. This introduces a different structure of the energy levels at the bottom of the conduction and top of the valence band in zigzag and armchair GQD which manifests itself in the oscillation of the energy gap with increasing size. The evolution of the bandgap with the number of carbon atoms is compared with the notion of confined Dirac Fermions and tested against {\it ab-initio} calculations of Kohn-Sham and TD-DFT energy gaps.

We study the practical performance of quantum-inspired algorithms for recommendation systems and linear systems of equations. These algorithms were shown to have an exponential asymptotic speedup compared to previously known classical methods for problems involving low-rank matrices, but with complexity bounds that exhibit a hefty polynomial overhead compared to quantum algorithms. This raised the question of whether these methods were actually useful in practice. We conduct a theoretical analysis aimed at identifying their computational bottlenecks, then implement and benchmark the algorithms on a variety of problems, including applications to portfolio optimization and movie recommendations. On the one hand, our analysis reveals that the performance of these algorithms is better than the theoretical complexity bounds would suggest. On the other hand, their performance as seen in our implementation degrades noticeably as the rank and condition number of the input matrix are increased. Overall, our results indicate that quantum-inspired algorithms can perform well in practice provided that stringent conditions are met: low rank, low condition number, and very large dimension of the input matrix. By contrast, practical datasets are often sparse and high-rank, precisely the type that can be handled by quantum algorithms., A popular summary can be found at https://medium.com/xanaduai/everything-you-always-wanted-to-know-about-quantum-inspired-algorithms-38ee1a0e30ef . Source code is available at https://github.com/XanaduAI/quantum-inspired-algorithms

We present here a theory and a computational tool, Silicon-{\sc Qnano}, to describe atomic scale quantum dots in Silicon. The methodology is applied to model dangling bond quantum dots (DBQDs) created on a passivated H:Si-(100)-(2$\times$1) surface by removal of a Hydrogen atom. The electronic properties of DBQD are computed by embedding it in a computational box of Silicon atoms. The surfaces of the computational box were constructed by using DFT as implemented in {\sc Abinit} program. The top layer was reconstructed by the formation of Si dimers passivated with H atoms while the bottom layer remained unreconstructed and fully saturated with H atoms. The computational box Hamiltonian was approximated by a tight-binding (TB) Hamiltonian by expanding the electron wave functions as a Linear Combination of Atomic Orbitals and fitting the bandstructure to {\it ab-initio} results. The parametrized TB Hamiltonian was used to model large finite Si(100) boxes (slabs) with number of atoms exceeding present capabilities of {\it ab-initio} calculations. The removal of one hydrogen atom from the reconstructed surface resulted in a DBQD state with wave function strongly localized around the Si atom and energy in the silicon bandgap. The DBQD could be charged with zero, one and two electrons. The Coulomb matrix elements were calculated and the charging energy of a two electron complex in a DBQD obtained., 20 Pages, 10 Figures

The electron localization function (ELF) by Becke and Edgecombe [A.D. Becke, K.E. Edgecombe, J. Chem. Phys. 92, 5397 (1990)] is routinely adopted as a descriptor of atomic shells and covalent bonds. Since the ELF and its related quantities find useful exploitation also in the construction of modern density functionals, the interest in complementing the ELF is linked to both the quests of improving electronic structure descriptors and density functional approximations. The ELF uses information which is available by considering parallel-spin electron pairs in single-reference many-body states. In this work, we complement this construction with information obtained by considering antiparallel-spin pairs whose short-range correlations are modeled by a density functional approximation. As a result, the approach requires only a contained computational effort. Applications to a variety of systems show that, in this way, we gain a spatial description of the bond in H2 (which is not available with the ELF) together with some trends not optimally captured by the ELF in other prototypical situations.

Atomically thin semiconducting transition-metal dichalcogenides enable new insight into physics of many-body effects mediated by Coulomb and electron-phonon interactions. We report photoluminescence (PL) and reflectivity contrast (RC) measurements of excitons ($X$) and trions ($T$) and Raman spectra of phonons in monolayers (MLs) of $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys with sulfur mole content up to $y=0.5$. We find the phonon energy crossing the trion binding energy with the sulfur mole content. Binary ${\mathrm{MoSe}}_{2}$ and ternary $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys exhibit contrasting behavior in the temperature evolution of excitons and trions PL intensity from $T=7$ to 295 K. In ${\mathrm{MoSe}}_{2}$, the trion dominates PL spectra at low temperatures but the exciton dominates PL at higher temperature. In contrast, in ternary $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys trions dominate PL spectra at all measured temperatures, with the trion to exciton PL intensity ratio increasing with sulfur content. We attribute the strong increase of the trion PL intensity in $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ MLs with increase of sulfur mole content to two effects: (i) strong increase of exciton-trion coupling mediated by the optical phonon, which is realized by tuning phonon energy through trion binding energy, and (ii) significant increase of two-dimensional electron gas concentration. We also demonstrate that increasing sulfur content in $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys increases total PL intensity at high temperature. With the temperature growth from 7 to 295 K, the total PL in ${\mathrm{MoSe}}_{2}$ decreases about three orders of magnitude more than in $\mathrm{Mo}{({\mathrm{S}}_{0.5}{\mathrm{Se}}_{0.5})}_{2}$.

Resonance energy transfer (RET) is an inherently anisotropic process. Even the simplest, well-known F\"orster theory, based on the transition dipole-dipole coupling, implicitly incorporates the anisotropic character of RET. In this theoretical work, we study possible signatures of the fundamental anisotropic character of RET in hybrid nanomaterials composed of a semiconductor nanoparticle (NP) decorated with molecular dyes. In particular, by means of a realistic kinetic model, we show that the analysis of the dye photoluminescence difference for orthogonal input polarizations reveals the anisotropic character of the dye-NP RET which arises from the intrinsic anisotropy of the NP lattice. In a prototypical core/shell wurtzite CdSe/ZnS NP functionalized with cyanine dyes (Cy3B), this difference is predicted to be as large as 75\% and it is strongly dependent in amplitude and sign on the dye-NP distance. We account for all the possible RET processes within the system, together with competing decay pathways in the separate segments. In addition, we show that the anisotropic signature of RET is persistent up to a large number of dyes per NP., Comment: 9 pages, 5 figures. Supplementary information available at http://pubs.rsc.org/en/content/articlelanding/2016/ra/c6ra22433d/unauth#!divAbstract

The challenge of simulating many-body models with analogue physical systems requires both experimental precision and very low operational temperatures. Atomically precise placement of dopants in Si permits the construction of nanowires by design. We investigate the suitability of these interacting electron systems as simulators of a fermionic extended Hubbard model on demand. We describe the single-particle wavefunctions as a linear combination of dopant orbitals (LCDO). The electronic states are calculated within configuration interaction (CI). Due to the peculiar oscillatory behavior of each basis orbital, properties of these chains are strongly affected by the interdonor distance R0, in a non-monotonic way. Ground state (T = 0 K) properties such as charge and spin correlations are shown to remain robust under temperatures up to 4 K for specific values of R0. The robustness of the model against disorder is also tested, allowing some fluctuation of the placement site around the target position. We suggest that finite donor chains in Si may serve as an analog simulator for strongly correlated model Hamiltonians. This simulator is, in many ways, complementary to those based on cold atoms in optical lattices—the trade-off between the tunability achievable in the latter and the survival of correlation at higher operation temperatures for the former suggests that both technologies are applicable for different regimes. Precisely positioned impurities in silicon could perform quantum simulations of models that are intractable for classical computers. Amintor Dusko and co-workers from Brazil’s Universidade Federal do Rio de Janeiro and the University of Ottawa in Canada have shown numerically that if phosphorus atoms in silicon devices are arranged in a line, their electrons can provide an experimental realization of the many-body Fermi-Hubbard model. This model, first presented in 1963, provides a heavily simplified description of high-temperature superconductors. However, a theoretical solution cannot be obtained for large systems. By simulating the model experimentally, Dusko et al.’s proposed system circumvents these computational limitations, and could reach temperature regimes that are not accessible with state-of-the-art cold atom simulators. Successful experimental realization could therefore provide new insights into a model that has been intensively studied since the 1960s.

The UV absorption spectrum of C-60 in n-hexane solvent has been revised by means of numerical analysis and time-dependent density functional theory (TDDFT). The absorption spectrum in the range 3-7 eV has been fitted by a spectral function that includes fourteen transitions with Gaussian lineshape, providing reference transition energies and intensities. The agreement between the experimental and theoretical UV absorption intensities has been considerably improved with respect to previous calculations, by including the solvent dielectric response via the polarizable continuum model (PCM). (C) 2014 Elsevier B.V. All rights reserved.

We develop a novel approach to treat excitation energy transfer in hybrid nanosystems composed by an organic molecule attached to a semiconductor nanoparticle. Our approach extends the customary Förster theory by considering interaction between transition multipole moments of the nanoparticle at all orders and a point-like transition dipole moment representing the molecule. Optical excitations of the nanoparticle are described through an envelope-function configuration interaction method for a single electron-hole pair. We applied the method to the prototypical case of a core/shell CdSe/ZnS semiconductor quantum dot which shows a complete suppression of the energy transfer for specific transitions which could not be captured by Förster theory.

The Polarizable Continuum Model (PCM) can be used in conjunction with Density Functional Theory (DFT) and its time-dependent extension (TDDFT) to simulate the electronic and optical properties of molecules and nanoparticles immersed in a dielectric environment, typically liquid solvents. In this contribution, we develop a methodology to account for solvation effects in real-space (and real-time) (TD)DFT calculations. The boundary elements method is used to calculate the solvent reaction potential in terms of the apparent charges that spread over the Van der Waals solute surface. In a real-space representation this potential may exhibit a Coulomb singularity at grid points that are close to the cavity surface. We propose a simple approach to regularize such singularity by using a set of spherical Gaussian functions to distribute the apparent charges. We have implemented the proposed method in the Octopus code and present results for the electrostatic contribution to the solvation free energies and solvatochromic shifts for a representative set of organic molecules in water., Version submitted to The Journal of Chemical Physics 10 pages, 7 Figures + supplemental material

A novel dyad composed of a diaminoterephthalate scaffold, covalently linked to a fullerene derivative, is explored as a nanosized charge separation unit powered by solar energy. Its opto-electronic properties are studied and the charge separation rate is determined. Simulations of the coupled electronic and nuclear dynamics in the Ehrenfest approximation are carried out on a sub 100 fs time scale after photoexcitation in order to gain insights about the mechanisms driving the charge separation. In particular, the role of vibronic coupling and of the detailed morphology are highlighted.

We estimate the effects of same-spin dynamical correlations on the electron localization by means of a simple modification of the electron localization function.

A general methodology is presented to model the opto-electronic properties of a dye molecule in the presence of a semiconductor nanoparticle (NP), a model system for the architecture of dye-sensitized solar cells. The method is applied to the L0 organic dye solvated with acetonitrile in the neighborhood of a TiO2 NP. The total reaction potential due to the polarization of the solvent and the metal oxide is calculated by extending the polarizable continuum model integral equation formalism. The ground state energy is computed by using density functional theory (DFT) while the vertical electronic excitations are obtained by time-dependent DFT in a state-specific corrected linear response scheme. We calculate the excited state oxidation potential (ESOP) for the protonated and deprotonated forms of the L0 dye at different distances and configurations with respect to the NP surface. The stronger renormalizations of the ESOP values due to the presence of the TiO2 nanostructure are found for the protonated dye, reaching a maximum of about -0.15 eV. The role of protonation effect is discussed in terms of the atomic Lowdin charges of the oxidized and reduced species. On the other hand, we observed a weak effect on the L0 optical excitation gap due to the polarization response of the NP. (C) 2013 AIP Publishing LLC.

Objective: The objective of this study is to compare a computerized deep brain stimulation (DBS) screening module (Comparing Private Practice vs. Academic Centers in Selection of DBS Candidates [COMPRESS], NeuroTrax Corp., Bellaire, TX, USA) with traditional triage by a movement disorders specialized neurologist as the gold standard. Methods: The COMPRESS consists of a combination of the Florida Surgical Questionnaire for Parkinson disease (FLASQ-PD), a cognitive assessment battery provided by MindStreams® (NeuroTrax Corp.), and the Geriatric Depression Scale and the Zung Anxiety Self-Assessment Scale. COMPRESS resulted in the classification of patients into three categories: “optimal candidate,”“probable candidate,” and “not a good candidate.” Similar categorical ratings made by a referring private practice neurologist and by a trained movement disorders specialist were compared with the ratings generated by COMPRESS. Results: A total of 19 subjects with Parkinson's disease were enrolled from five private neurological practices. The clinical impressions of the private practice neurologist vs. those of the movement disorders specialist were in agreement approximately half the time (10/19 cases). The movement disorders specialist and COMPRESS agreed on 15/19 cases. A further comparison between outcomes from the entire COMPRESS module and the FLASQ-PD questionnaire by itself resulted in high agreement (18/19 cases in agreement). Conclusions: The COMPRESS agreed with an in-person evaluation by a movement disorders neurologist approximately 80% of the time. The computerized COMPRESS did not provide any screening advantage over the short FLASQ-PD paper questionnaire. Larger studies will be needed to assess the utility and cost effectiveness of this computerized triage method for DBS.

We report on ab-initio calculations of the electronic structure and optical absorption response of the black dye sensitizer in gas phase. We show that, despite the large size of this molecule, the second-order multiconfiguration quasi-degenerate perturbation theory (MC-QDPT) can be used to calculate vertical excitation energies, oscillator strengths and optical absorption spectra. The zeroth-order reference states entering perturbation calculations are complete active space (CAS) configuration interaction (CI) wave functions computed for 12 active electrons distributed in 12 active orbitals. We found that the CI approach is not enough for taking into account the strong dynamical correlation effects in this system. In fact, the excitation energies of the CAS-CI target states are strongly renormalized by the MC-QDPT calculations. In the calculated absorption spectra, the analysis of the perturbed wavefunctions revealed that the stronger absorption bands correspond to metal-to-ligand and ligand-to-ligand charge transfer processes. Comparison with independent time-dependent extension (TDDFT) calculations performed with different functionals shows that corrections to the long-range behavior of the functional is pivotal to achieve agreement with the MC-QDPT results.

We report exact diagonalization studies of inelastic light scattering in few-electron quantum dots under the strong confinement regime characteristic of self-assembled dots. We apply the orthodox (second-order) theory for scattering due to electronic excitations, leaving for the future the consideration of higher-order effects in the formalism (phonons, for example), which seem relevant in the theoretical description of available experiments. Our numerical results stress the dominance of monopole peaks in Raman spectra and the breakdown of selection rules in open-shell dots. The dependence of these spectra on the number of electrons in the dot and the incident photon energy is explicitly shown. Qualitative comparisons are made with recent experimental results., 11 pages, 11 figures

In a two-dimensional parabolic quantum dot charged with $N$ electrons, Thomas-Fermi theory states that the ground-state energy satisfies the following non-trivial relation: $E_{gs}/(\hbar\omega)\approx N^{3/2} f_{gs}(N^{1/4}\beta)$, where the coupling constant, $\beta$, is the ratio between Coulomb and oscillator ($\hbar\omega$) characteristic energies, and $f_{gs}$ is a universal function. We perform extensive Configuration Interaction calculations in order to verify that the exact energies of relatively large quantum dots approximately satisfy the above relation. In addition, we show that the number of energy levels for intraband and interband (excitonic and biexcitonic) excitations of the dot follows a simple exponential dependence on the excitation energy, whose exponent, $1/\Theta$, satisfies also an approximate scaling relation {\it a la} Thomas-Fermi, $\Theta/(\hbar\omega)\approx N^{-\gamma} g(N^{1/4}\beta)$. We provide an analytic expression for $f_{gs}$, based on two-point Pad\'e approximants, and two-parameter fits for the $g$ functions., Comment: final version, to appear in Phys. Rev. B

In a quantum dot with dozens of electrons, an approximation beyond Tamm-Dankoff is used to construct the quantum states with an additional electron-hole pair, i.e. the "excitonic" states. The lowest states mimic the non-interacting spectrum, but with excitation gaps renormalized by Coulomb interactions. At higher excitation energies, the computed density of energy levels shows an exponential increase with energy. In the interband absorption, we found a background level in the quasicontinuum of states rising linearly with the excitation energy. Above this background, there are distinct peaks related to single resonances or to groups of many states with small interband dipole moments.

A theoretical description of electronic Raman scattering from GaAs/AlxGa1 xAs artificial atoms under the influence of an external magnetic field is presented. Raman spectra with laser excitation energy in the interval Egap 30 meV to Egap are computed in the polarized and depolarized geometry. The polarization ratios for the collective and single-particle excitations indicate a breakdown of the Raman polarization selection rules once the magnetic field is switched on. A Raman intensity jump rule at the band gap is predicted in our calculations. This rule can be a useful tool for identifying the physical nature (charge or spin) of the electronic excitations in quantum dots in low magnetic fields. a few-electron CDEs in InGaAs self-assembled quantum dots by means of Raman spectroscopy. In this paper, we compute the near-resonance Raman spectrum of a medium-size disk-shaped quantum dot con- taining 42 electrons in an external magnetic field applied perpendicular to the plane of the dot. The method and approximations used to obtain the many-body wave func- tions and energies of the required states are explained in Ref. (7). Energies and wave-functions of the final ex- cited states are obtained by means of the random phase approximation 8 (RPA). The calculation of the monopole operator matrix elements between each final state and the ground state, allows the identification of the collec- tive modes (CDEs, SDEs) and the SPEs. By computing the Raman polarization ratios we evaluate the extent to which the magnetic field breaks down the polarization se- lection rules for both collective and SPEs. We report the first observation of what we term the Raman intensity jump-rule. This effect, more clearly seen for the collec- tive modes, can be enunciated as follows: a monotonic increase of a Raman peak intensity when the incident laser energy is increased from below up to the effective band-gap of the dot in the polarized (depolarized) geom- etry indicates that the excited state associated with that peak is a charge (spin) excitation. If we look at the same state in its opposite geometry, we find quite small values of the Raman intensities at any incident energy except at the effective band-gap, in which case a sudden jump of the Raman intensity should be detected. From theory 9 , we know that the Raman peak inten- sity is proportional to the transition amplitude squared, |Afi| 2 , where

We present theoretical results concerning inelastic light (Raman) scattering from semiconductor quantum dots. The characteristics of each dot state (whether it is a collective or single-particle excitation, its multipolarity, and its spin) are determined independently of the Raman spectrum, in such a way that common beliefs used for level assignments in experimental spectra can be tested. We explore the usefulness of below band gap excitation and an external magnetic field to identify charge and spin excited states of a collective or single-particle nature., To appear in a special issue of Solid State Communications dedicated to Eli Burstein

Inelastic (Raman) light scattering intensities for a 42-electron quantum dot under off-resonance conditions and in different spin and angular momentum channels are computed in order to test whether final collective states become the dominant peaks in the process. The results of the calculations set a limit to the spread use of the off-resonant approximation for the theoretical description of Raman processes.

A consistent semiquantitative theoretical analysis of electronic Raman scattering from many-electron quantum dots under resonance excitation conditions has been performed. The theory is based on random-phase-approximation-like wave functions, with the Coulomb interactions treated exactly, and hole valence-band mixing accounted for within the Kohn-Luttinger Hamiltonian framework. The widths of intermediate and final states in the scattering process, although treated phenomenologically, play a significant role in the calculations, particularly for well above band gap excitation. The calculated polarized and unpolarized Raman spectra reveal a great complexity of features and details when the incident light energy is swept from below, through, and above the quantum dot band gap. Incoming and outgoing resonances dramatically modify the Raman intensities of the single particle, charge density, and spin density excitations. The theoretical results are presented in detail and discussed with regard to experimental observations., Comment: Submitted to Phys. Rev. B

A consistent calculation of resonant inelastic (Raman) scattering amplitudes for relatively large quantum dots, which takes account of valence-band mixing, discrete character of the spectrum in intermediate and final states, and interference effects, is presented. Raman peaks in charge and spin channels are compared with multipole strengths and with the density of energy levels in final states. A qualitative comparison with the available experimental results is given., Comment: 5 pages, accepted in J. Phys.: Condens. Matter

Resonant inelastic (Raman) light scattering off neutral GaAs quantum dots which contain a mean number, N=42, of electron-hole pairs is computed. We find Raman amplitudes corresponding to strongly collective final states (charge-density excitations) of similar magnitude as the amplitudes related to weakly collective or single-particle excitations. As a function of the incident laser frequency or the magnetic field, they are rapidly varying amplitudes. It is argued that strong Raman peaks should come out in the spin-density channels, not related to valence-band mixing effects in the intermediate states., Comment: Accepted in Physical Review B

The structure of E1 resonances is examined in a microscopic random phase approximation calculation for neutral, symmetric, closed shell, electron-hole systems in a quantum dot. The number of electron-hole pairs, N, is varied from 6 to 42. The ocurrence of small, but distinct, E1 peaks in the far infrared spectra located in the low energy tail of the giant dipole resonance and consisting of highly coherent electron-hole excitations is predicted. These pigmy resonances account for about 2 % of the dipole energy-weighted photoabsorption sum rule. A very weak dependence of the average pigmy resonance energy on the number of electron-hole pairs is found., Comment: LaTeX, 4 pages, 2 figures, submitted to Phys.Rev.A

A resonance behaviour of the far infrared absorption probability at a frequency \sim N^{1/4} is predicted for clusters of N electron-hole pairs (2\le N\le 110) confined in disk-shaped quantum dots. For radially symmetric dots, the absorption is dominated by a Giant Dipole Resonance, which accounts for more than 98 % of the energy-weighted photoabsorption sum rule., final version

We find an unexpected scaling in the correlation energy of artificial atoms, i.e., harmonically confined two-dimensional quantum dots. The scaling relation is found through extensive numerical examinations including Hartree-Fock, variational quantum Monte Carlo, density-functional, and full configuration-interaction calculations. We show that the correlation energy, i.e., the true ground-state total energy subtracted by the Hartree-Fock total energy, follows a simple function of the Coulomb energy, confimenent strength and, the number of electrons. We find an analytic expression for this function, as well as for the correlation energy per particle and for the ratio between the correlation and total energies. Our tests for independent diffusion Monte Carlo and coupled-cluster results for quantum dots -- including open-shell data -- confirm the generality of the obtained scaling. As the scaling is also well applicable to $\gtrsim$ 100 electrons, our results give interesting prospects for the development of correlation functionals within density-functional theory., Comment: Accepted to Journal of Physics: Condensed Matter