arXiv subjects

Xavier Blase

Publications and source records attributed to Xavier Blase.

At least 19 recordsLinked to original sources

arXiv2026

Stability and optoelectronic properties of oligothiophene molecules confined in boron-nitride nanotubes : A many-body theoretical approach

We investigate the structural and optoelectronic properties of oligothiophene (nT) molecules encapsulated in boron-nitride (BN) nanotubes using density functional theory, many-body GW and Bethe-Salpeter equation approaches. We show that the binding energy is maximized for tube diameters of approximately 10 {\AA}, decreasing gradually for larger diameters. The nT molecules can slide along the tube with a corrugation potential smaller than room temperature thermal energy, promoting their head-to-tail aggregation. Regarding the electronic properties, we find that hybridization with the tube electronic states has a much smaller effect than that of screening, which can close the nT photoemission gap by as much as an eV. A simple model for the polarization of the BN tube demonstrates how these polarization effects decrease with increasing nT molecule length and BN tube diameter. Compared to the gas phase optical properties, structural relaxation, hybridization, and screening can redshift the absorption onset by up to 250 meV for isolated intercalated nTs. Additionally, the study of a head-to-tail sexithiophene dimer reveals an exciton-exciton interaction that splits the absorption onset into a lowest bright exciton, separated by approximately 75 meV from a dark peak. This suggests possible collective effects upon the formation of nT chains inside BN tubes. Our results confirm and clarify experimental data, mitigating the conclusion that insulating BN tubes just act as a protecting environment for encapsulated molecules.

cond-mat.mes-hall↗

arXiv2026

Efficient analytic continuation approach to Bethe-Salpeter excitation spectra in selected energy windows

We explore the merits of building the Bethe-Salpeter absorption spectrum in a specific energy range using analytic continuation techniques. Specifically, we calculate iteratively a few $\bar{\bar \alpha}(z_k)$ polarizability tensors for a coarse set of $(z_k)$ frequencies in the complex-plane. These data allow constructing a continued-fraction representation for $\bar{\bar{\alpha}}(z)$ that is used to calculate the absorption spectrum close to the real energy axis in the desired energy range. The number and location of these sampling complex frequencies are discussed. The importance of building a continued-fraction representation of the full polarizability tensor with matrix-valued coefficients is emphasized. We show how to extract the poles of the continued fraction as a tool for analyzing the resulting spectra. We study as examples the valence excitations of a paradigmatic dipeptide, the C$_{60}$ fullerene and its PCBM derivative, together with the description of the surface plasmon resonance of the Ag$_{20}$ silver cluster. Further, the high-energy C$_{60}$ X-ray absorption spectrum is explored.

cond-mat.mtrl-sci↗

arXiv2025

Optical spectra of small silver clusters with the Bethe-Salpeter formalism: a Reassessment

We study the optical absorption spectra of small Ag_n (n=2,4,6,8) clusters using the Bethe-Salpeter equation (BSE) formalism with a Hamiltonian built from GW quasiparticle energies. Calculations are based on an effective core potential including the 4sp shells in the valence. Non-self-consistent G0W0 calculations relying on input Kohn-Sham eigenstates generated with semilocal functionals lead to very poor BSE absorption spectra, confirming a previous observation. However, eigenvalue or quasiparticle self-consistent GW calculations dramatically improves the agreement with experiments and TD-DFT spectra obtained with range-separated hybrid functionals. The importance of the position of the 4d occupied band is emphasized.

cond-mat.mtrl-sci↗

arXiv2024

Joint Approximate Diagonalization approach to Quasiparticle Self-Consistent $GW$ calculations

We introduce an alternative route to quasiparticle self-consistent $GW$ calculations ($\mathrm{qs}GW$) on the basis of a Joint Approximate Diagonalization of the one-body $GW$ Green's functions $G(\varepsilon_n^{QP})$ taken at the input quasiparticle energies. Such an approach allows working with the full dynamical self-energy, without approximating the latter by a symmetrized static form as in the standard $\mathrm{qs}GW$ scheme. Calculations on the $GW$100 molecular test set lead nevertheless to a good agreement, at the 65 meV mean-absolute-error accuracy on the ionization potential, with respect to the conventional $\mathrm{qs}GW$ approach. We show further that constructing the density matrix from the full Green's function as in the fully self-consistent $\mathrm{sc}GW$ scheme, and not from the occupied quasiparticle one-body orbitals, allows obtaining a scheme intermediate between $\mathrm{qs}GW$ and $\mathrm{sc}GW$ approaches, closer to CCSD(T) reference values.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous propagators and the particle-particle channel: Bethe-Salpeter equation

The Bethe-Salpeter equation has been extensively employed to compute the two-body electron-hole propagator and its poles which correspond to the neutral excitation energies of the system. Through a different time-ordering, the two-body Green's function can also describe the propagation of two electrons or two holes. The corresponding poles are the double ionization potentials and double electron affinities of the system. In this work, a Bethe-Salpeter equation for the two-body particle-particle propagator is derived within the linear-response formalism using a pairing field and anomalous propagators. This framework allows us to compute kernels corresponding to different self-energy approximations ($GW$, $T$-matrix, and second-Born) as in the usual electron-hole case. The performance of these various kernels is gauged for singlet and triplet valence double ionization potentials using a set of 23 small molecules. The description of double core hole states is also analyzed.

physics.chem-ph↗

arXiv2024

Polarizable Continuum Models and Green's Function $\bf{GW}$ Formalism: On the Dynamics of the Solvent Electrons

The many-body $GW$ formalism, for the calculation of ionization potentials or electronic affinities, relies on the frequency-dependent dielectric function built from the electronic degrees of freedom. Considering the case of water as a solvent treated within the polarizable continuum model, we explore the impact of restricting the full frequency-dependence of the solvent electronic dielectric response to a frequency-independent $(\epsilon_\infty)$ optical dielectric constant. For solutes presenting small to large highest-occupied to lowest-unoccupied molecular orbital energy gaps, we show that such a restriction induces errors no larger than a few percent on the energy level shifts from the gas to the solvated phase. We further introduce a remarkably accurate single-pole model for mimicking the effect of the full frequency dependence of the water dielectric function in the visible-UV range. This allows a fully dynamical embedded $GW$ calculation with the only knowledge of the cavity reaction field calculated for the $\epsilon_\infty$ optical dielectric constant.

cond-mat.mtrl-sci↗

arXiv2024

Reference CC3 Excitation Energies for Organic Chromophores: Benchmarking TD-DFT, BSE/$GW$ and Wave Function Methods

To expand the QUEST database of highly-accurate vertical transition energies, we consider a series of large organic chromogens ubiquitous in dye chemistry, such as anthraquinone, azobenzene, BODIPY, and naphthalimide. We compute, at the CC3 level of theory, the singlet and triplet vertical transition energies associated with the low-lying excited states. This leads to a collection of more than 120 new highly-accurate excitation energies. Subsequently, we employ these reference values to benchmark a series of lower-order wave function approaches, including the popular ADC(2) and CC2 schemes, as well as time-dependent density-functional theory (TD-DFT), both with and without applying the Tamm-Dancoff approximation (TDA). At the TD-DFT level, we evaluate a large panel of global, range-separated, local, and double hybrid functionals. Additionally, we assess the performance of the Bethe-Salpeter equation (BSE) formalism relying on both $G_0W_0$ and ev$GW$ quasiparticle energies evaluated from various starting points. It turns out that CC2 and ADC(2.5) are the most accurate models amongst those with respective $\mathcal{O}(N^5)$ and $\mathcal{O}(N^6)$ scalings with system size. In contrast, CCSD does not outperform CC2. The best performing exchange-correlation functionals include BMK, M06-2X, M06-SX, CAM-B3LYP, $\omega$B97X-D, and LH20t, with average deviations of approximately 0.20 eV or slightly below. Errors on vertical excitation energies can be further reduced by considering double hybrids. Both SOS-$\omega$B88PP86 and SOS-$\omega$PBEPP86 exhibit particularly attractive performances with overall quality on par with CC2, whereas PBE0-DH and PBE-QIDH are only slightly less efficient. BSE/ev$GW$ calculations based on Kohn-Sham starting points have been found to be particularly effective for singlet transitions, but much less for their triplet counterparts.

physics.chem-ph↗

arXiv2024

From many-body ab initio to effective excitonic models: a versatile mapping approach including environmental embedding effects

We present an original multi-state projective diabatization scheme based on the Green's function formalism that allows the systematic mapping of many-body ab initio calculations onto effective excitonic models. This method inherits the ability of the Bethe-Salpeter equation to describe Frenkel molecular excitons and intermolecular charge-transfer states equally well, as well as the possibility for an effective description of environmental effects in a QM/MM framework. The latter is found to be a crucial element in order to obtain accurate model parameters for condensed phases and to ensure their transferability to excitonic models for extended systems. The method is presented through a series of examples illustrating its quality, robustness, and internal consistency.

physics.chem-ph↗

arXiv2024

Static versus dynamically polarizable environments within the many-body $\bf{GW}$ formalism

Continuum or discrete polarizable models for the study of optoelectronic processes in embedded subsystems rely mostly on the restriction of the surrounding electronic dielectric response to its low frequency limit. Such a description hinges on the assumption that the electrons in the surrounding medium react instantaneously to any excitation in the central subsystem, treating thus the environment in the adiabatic limit. Exploiting a recently developed embedded $GW$ formalism, with an environment described at the fully ab initio level, we assess the merits of the adiabatic limit with respect to an environment where the full dynamics of the dielectric response is considered. Further, we show how to properly take the static limit of the environment susceptibility, introducing the so-called Coulomb-hole and screened-exchange contributions to the reaction field. As a first application, we consider a C$_{60}$ molecule at the surface of a C$_{60}$ crystal, namely a case where the dynamics of the embedded and embedding subsystems are similar. The common adiabatic assumption, when properly treated, generates errors below $10\%$ on the polarization energy associated with frontier energy levels and associated energy gaps. Finally, we consider a water molecule inside a metallic nanotube, the worst case for the environment adiabatic limit. The error on the gap polarization energy remains below $10\%$, even though the error on the frontier orbitals polarization energies can reach a few tenths of an electronvolt.

cond-mat.mtrl-sci↗

arXiv2023

Many-body $GW$ calculations with very large scale polarizable environments made affordable: a fully ab initio QM/QM approach

We present a many-body $GW$ formalism for quantum subsystems embedded in discrete polarizable environments containing up to several hundred thousand atoms described at a fully ab initio random phase approximation level. Our approach is based on a fragment approximation in the construction of the Green's function and independent-electron susceptibilities. Further, the environing fragments susceptibility matrices are reduced to a minimal but accurate representation preserving low order polarizability tensors through a constrained minimization scheme. This approach dramatically reduces the cost associated with inverting the Dyson equation for the screened Coulomb potential $W$, while preserving the description of short to long-range screening effects. The efficiency and accuracy of the present scheme is exemplified in the paradigmatic cases of fullerene bulk, surface, subsurface, and slabs with varying number of layers.

physics.chem-ph↗

arXiv2023

Multiorbital exciton formation in an organic semiconductor

Harnessing the optoelectronic response of organic semiconductors requires a thorough understanding of the fundamental light-matter interaction that is dominated by the excitation of correlated electron-hole pairs, i.e. excitons. The nature of these excitons would be fully captured by knowing the quantum-mechanical wavefunction, which, however, is difficult to access both theoretically and experimentally. Here, we use femtosecond photoemission orbital tomography in combination with many-body perturbation theory to gain access to exciton wavefunctions in organic semiconductors. We find that the coherent sum of multiple electron-hole pair contributions that typically make up a single exciton can be experimentally evidenced by photoelectron spectroscopy. For the prototypical organic semiconductor buckminsterfullerene (C$_{60}$), we show how to disentangle such multiorbital contributions and thereby access key properties of the exciton wavefunctions including localization, charge-transfer character, and ultrafast exciton formation and relaxation dynamics.

cond-mat.mes-hall↗

arXiv2022

Universal polarization energies for defects in monolayer, surface and bulk hexagonal boron nitride : A finite-size fragments GW approach

We study defect energy levels in hexagonal boron-nitride with varying number of layers using a fragment many-body $GW$ formalism, taking as examples the paradigmatic carbon-dimer and $C_BV_N$ defects. We show that a single layer can be fragmented in polarizable finite-size areas reproducing faithfully the effect of the dielectric environment, dramatically facilitating the study at the many-body level of point defects in the dilute limit. The evolution of defect energy levels from the monolayer to a $n$-layer system due to increased screening, labeled polarization energies, follow a simple $({ΔP}/n + P_{\infty})$ behavior. The coefficients $ΔP$ and $P_{\infty}$ are found to be close-to-universal, with opposite signs for holes and electrons, characterizing mainly the host and the position of the defect (surface or bulk), but hardly the defect type. Our results rationalize the evolution of defect energy levels with layers number, allowing to safely extrapolate results obtained for the monolayer to few-layers, surface or bulk \textit{h}-BN. The present many-body fragment approach further opens the door to studying disordered 2D layers.

cond-mat.mtrl-sci↗

arXiv2021

Photoluminescent properties of the carbon-dimer defect in hexagonal boron-nitride: a many-body finite-size cluster approach

We study the carbon dimer defect in a hexagonal boron-nitride monolayer using the GW and Bethe-Salpeter many-body perturbation theories within a finite size cluster approach. While quasiparticle energies converge very slowly with system size due to missing long-range polarization effects, optical excitations converge much faster, with a $1/R^3$ scaling law with respect to cluster average radius. We obtain a luminescence zero-phonon energy of 4.36 eV, including significant 0.13 eV zero-point vibrational energy and 0.15 eV reorganization energy contributions. Inter-layer screening decreases further the emission energy by about 0.3 eV. These results bring support to the recent identification of the substitutional carbon dimer as the likely source of the zero-phonon 4.1 eV luminescence line. Finally, the GW quasiparticle energies are extrapolated to the infinite h-BN monolayer limit, leading to a predicted defect HOMO-LUMO photoemission gap of 7.6 eV. Comparison with the optical gap yields a very large excitonic binding energy of 3 eV for the associated localized Frenkel exciton.

cond-mat.mes-hall↗

arXiv2021

Doping-induced dielectric catastrophe prompts free-carrier release in organic semiconductors

The control over material properties attainable through molecular doping is essential to many technological applications of organic semiconductors, such as OLED or thermoelectrics. These excitonic semiconductors typically reach the degenerate limit only at impurity concentrations of 5-10\%, a phenomenon that has been put in relation to the strong Coulomb binding between charge carriers and ionized dopants, and whose comprehension remained elusive so far. This study proposes a general mechanism for the release of carriers at finite doping in terms of collective screening phenomena. A multiscale model for the dielectric properties of doped organic semiconductor is set up by combining first principles and microelectrostatic calculations. Our results predict a large nonlinear enhancement of the dielectric constant (ten-fold at 8\% load) as the system approaches a dielectric instability (catastrophe) upon increasing doping. This can be attributed to the presence of highly polarizable host-dopant complexes, plus a nontrivial leading contribution from dipolar interactions in the disordered and heterogeneous system. The enhanced screening in the material drastically reduces the (free) energy barriers for electron-hole separation, rationalizing the possibility for thermal charge release. The proposed mechanism is consistent with conductivity data and sets the basis for achieving higher conductivities at lower doping loads.

cond-mat.mtrl-sci↗

arXiv2021

Cubic-scaling all-electron GW calculations with a separable density-fitting space-time approach

We present an implementation of the $GW$ space-time approach that allows cubic-scaling all-electron calculations with standard Gaussian basis sets without exploiting any localization nor sparsity considerations. The independent-electron susceptibility is constructed in a time representation over a non-uniform distribution of real-space locations $\lbrace {\bf r}_k \rbrace$ optimized within a separable resolution-of-the-identity framework to reproduce standard Coulomb-fitting calculations with meV accuracy. The compactness of the obtained $\lbrace {\bf r}_k \rbrace$ distribution leads to a crossover with the standard Coulomb-fitting scheme for system sizes below a few hundred electrons. The needed analytic continuation follows a recent approach that requires the continuation of the screened Coulomb potential rather than the much more structured self-energy. The present scheme is benchmarked over large molecular sets and scaling properties are demonstrated on a family of defected hexagonal boron-nitride flakes containing up to 6000 electrons.

physics.comp-ph↗

arXiv2021

Reference Energies for Intramolecular Charge-Transfer Excitations

In the aim of completing our previous efforts devoted to local and Rydberg transitions in organic compounds, we provide a series of highly-accurate vertical transition energies for intramolecular charge-transfer transitions occurring in ($π$-conjugated) molecular compounds. To this end we apply a composite protocol consisting of linear-response CCSDT excitation energies determined with Dunning's double-$ζ$ basis set corrected by CC3/CCSDT-3 energies obtained with the corresponding triple-$ζ$ basis. Further basis set corrections (up to \emph{aug}-cc-pVQZ) are obtained at the CCSD and CC2 level. We report 30 transitions obtained in 17 compounds. These reference values are then used to benchmark a series of wave function (CIS(D), SOPPA, RPA(D), EOM-MP2, CC2, CCSD, CCSD(T)(a)*, CCSDR(3), CCSDT-3, CC3, ADC(2), ADC(3), and ADC(2.5)), the Green's function-based Bethe-Salpeter equation (BSE) formalism performed on top of the partially self-consistent ev$GW$ scheme considering two different starting points (BSE/ev$GW$@HF and BSE/ev$GW$@PBE0), and TD-DFT combined with several exchange-correlation functionals (B3LYP, PBE0, M06-2X, CAM-B3LYP, LC-$ω$HPBE, $ω$B97X, $ω$B97X-D, and M11).

physics.chem-ph↗

arXiv2020

The influence of impurities on the charge carrier mobility of small molecule organic semiconductors

Amorphous organic semiconductors based on small molecules and polymers are used in many applications, most prominently organic light emitting diodes (OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in most currently available organic semiconductors and limit charge transport and thus device efficiency. The microscopic cause as well as the chemical nature of these traps are presently not well understood. Using a multiscale model we characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors. We use the model to quantitatively describe the influence of water molecules and water-oxygen complexes on the electron and hole mobilities. These species are seen to impact the shape of the density of states and to act as explicit charge traps within the energy gap. Our results show that trap states introduced by molecular oxygen can be deep enough to limit the electron mobility in widely used materials.

cond-mat.soft↗

arXiv2020

Dynamical Correction to the Bethe-Salpeter Equation Beyond the Plasmon-Pole Approximation

The Bethe-Salpeter equation (BSE) formalism is a computationally affordable method for the calculation of accurate optical excitation energies in molecular systems. Similar to the ubiquitous adiabatic approximation of time-dependent density-functional theory, the static approximation, which substitutes a dynamical (i.e., frequency-dependent) kernel by its static limit, is usually enforced in most implementations of the BSE formalism. Here, going beyond the static approximation, we compute the dynamical correction of the electron-hole screening for molecular excitation energies thanks to a renormalized first-order perturbative correction to the static BSE excitation energies. The present dynamical correction goes beyond the plasmon-pole approximation as the dynamical screening of the Coulomb interaction is computed exactly within the random-phase approximation. Our calculations are benchmarked against high-level (coupled-cluster) calculations, allowing to assess the clear improvement brought by the dynamical correction for both singlet and triplet optical transitions.

physics.chem-ph↗