Fast and Accurate Electronic Excitations in Cyanines with the Many-Body Bethe–Salpeter Approach

The accurate prediction of the optical signatures of cyanine derivatives remains an important challenge in theoretical chemistry. Indeed, up to now, only the most expensive quantum chemical methods (CAS-PT2, CC, DMC, etc.) yield consistent and accurate data, impeding the applications on real-life molecules. Here, we investigate the lowest lying singlet excitation energies of increasingly long cyanine dyes within the <i>GW</i> and Bethe–Salpeter Green’s function many-body perturbation theory. Our results are in remarkable agreement with available coupled-cluster (exCC3) data, bringing these two single-reference perturbation techniques within a 0.05 eV maximum discrepancy. By comparison, available TD-DFT calculations with various semilocal, global, or range-separated hybrid functionals, overshoot the transition energies by a typical error of 0.3–0.6 eV. The obtained accuracy is achieved with a parameter-free formalism that offers similar accuracy for metallic or insulating, finite size or extended systems

Modeling the Photochrome–TiO₂ Interface with Bethe–Salpeter and Time-Dependent Density Functional Theory Methods

Hybrid organic–inorganic semiconductor systems have important applications in both molecular electronics and photoresponsive materials. The characterizations of the interface and of the electronic excited-states of these hybrid systems remain a challenge for state-of-the-art computational methods, as the systems of interest are large. In the present investigation, we present for the first time a many-body Green’s function Bethe–Salpeter investigation of a series of photochromic molecules adsorbed onto TiO₂ nanoclusters. On the basis of these studies, the performance of time-dependent density functional theory (TD-DFT) calculations is assessed. In addition, the photochromic properties of different hybrid systems are also evaluated. This work shows that qualitatively different conclusions can be reached with TD-DFT relying on various exchange–correlation functionals for such organic–inorganic interfaces and paves the way to more accurate simulation of many hybrid materials

Calculations of <i>n</i>→π* Transition Energies: Comparisons Between TD-DFT, ADC, CC, CASPT2, and BSE/<i>GW</i> Descriptions

Using a large panel of theoretical approaches, namely, CC2, CCSD, CCSDR(3), CC3, ADC(2), ADC(3), CASPT2, time-dependent density functional theory (TD-DFT), and BSE/ev<i>GW</i>, the two latter combined with different exchange-correlation functionals, we investigate the lowest singlet transition in 23 <i>n</i>→π* compounds based on the nitroso, thiocarbonyl, carbonyl, and diazo chromophores. First, for 16 small derivatives we compare the transition energies provided by the different wave function approaches to define theoretical best estimates. For this set, it surprisingly turned out that ADC(2) offers a better match with CC3 than ADC(3). Next, we use 10 functionals belonging to the “LYP” and “M06” families and compare the TD-DFT and the BSE/ev<i>GW</i> descriptions. The BSE/ev<i>GW</i> results are less sensitive than their TD-DFT counterparts to the selected functional, especially in the M06 series. Nevertheless, BSE/ev<i>GW</i> delivers larger errors than TD-CAM-B3LYP, which provides extremely accurate results in the present case, especially when the Tamm–Dancoff approximation is applied. In addition, we show that, among the different starting points for BSE/ev<i>GW</i> calculations, M06-2X eigenstates stand as the most appropriate. Finally, we confirm that the trends observed on the small compounds pertain in larger molecules

Combining the Bethe–Salpeter Formalism with Time-Dependent DFT Excited-State Forces to Describe Optical Signatures: NBO Fluoroborates as Working Examples

We propose to use a blend of methodologies to tackle a challenging case for quantum approaches: the simulation of the optical properties of asymmetric fluoroborate derivatives. Indeed, these dyes, which present a low-lying excited-state exhibiting a cyanine-like nature, are treated not only with the Time-Dependent Density Functional Theory (TD-DFT) method to determine both the structures and vibrational patterns but also with the Bethe–Salpeter approach to compute both the vertical absorption and emission energies. This combination allows us to obtain 0–0 energies with a significantly improved accuracy compared to the “raw” TD-DFT estimates. We also discuss the impact of various declinations of the Polarizable Continuum Model (linear-response, corrected linear-response, and state-specific models) on the obtained accuracy

Benchmark Many-Body <i>GW</i> and Bethe–Salpeter Calculations for Small Transition Metal Molecules

We study the electronic and optical properties of 39 small molecules containing transition metal atoms and 7 others related to quantum-dots for photovoltaics. We explore in particular the merits of the many-body <i>GW</i> formalism, as compared to the ΔSCF approach within density functional theory, in the description of the ionization energy and electronic affinity. Mean average errors of 0.2–0.3 eV with respect to experiment are found when using the PBE0 functional for ΔSCF and as a starting point for <i>GW</i>. The effect of partial self-consistency at the <i>GW</i> level is explored. Further, for optical excitations, the Bethe–Salpeter formalism is found to offer similar accuracy as time-dependent DFT-based methods with the hybrid PBE0 functional, with mean average discrepancies of about 0.3 and 0.2 eV, respectively, as compared to available experimental data. Our calculations validate the accuracy of the parameter-free <i>GW</i> and Bethe–Salpeter formalisms for this class of systems, opening the way to the study of large clusters containing transition metal atoms of interest for photovoltaic applications

Few-Electron Edge-State Quantum Dots in a Silicon Nanowire Field-Effect Transistor

We investigate the gate-induced onset of few-electron regime through the undoped channel of a silicon nanowire field-effect transistor. By combining low-temperature transport measurements and self-consistent calculations, we reveal the formation of one-dimensional conduction modes localized at the two upper edges of the channel. Charge traps in the gate dielectric cause electron localization along these edge modes, creating elongated quantum dots with characteristic lengths of ∼10 nm. We observe single-electron tunneling across two such dots in parallel, specifically one in each channel edge. We identify the filling of these quantum dots with the first few electrons, measuring addition energies of a few tens of millielectron volts and level spacings of the order of 1 meV, which we ascribe to the valley orbit splitting. The total removal of valley degeneracy leaves only a 2-fold spin degeneracy, making edge quantum dots potentially promising candidates for silicon spin qubits