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Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

The optical spectra of two-dimensional (2D) periodic systems provide a challenge for time-dependent density-functional theory (TDDFT) because of the large excitonic effects in these materials. In this work we explore how accurately these spectra can be described within a pure Kohn-Sham time-dependent density-functional framework, i.e., a framework in which no theory beyond Kohn-Sham density-functional theory, such as $GW$, is required to correct the Kohn-Sham gap. To achieve this goal we adapted a recent approach we developed for the optical spectra of 3D systems [Cavo, Berger, Romaniello, Phys. Rev. B 101, 115109 (2020)] to those of 2D systems. Our approach relies on the link between the exchange-correlation kernel of TDDFT and the derivative discontinuity of ground-state density-functional theory, which guarantees a correct quasi-particle gap, and on a generalization of the polarization functional [Berger, Phys. Rev. Lett., 115, 137402 (2015)], which describes the excitonic effects. We applied our approach to two prototypical 2D monolayers, $h$-BN and MoS$_2$. We find that our protocol gives a qualitative good description of the optical spectrum of $h$-BN, whereas improvements are needed for MoS$_2$ to describe the intensity of the excitonic peaks.

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Aiming at completing the sets of FCI-quality transition energies that we recently developed (

This work presents a series of highly-accurate excited-state properties obtained using high-order coupled-cluster (CC) calculations performed with a series of diffuse containing basis sets, as well as extensive comparisons with experimental values. Indeed, we have computed both the main ground-to-excited transition property, the oscillator strength, as well as the ground- and excited-state dipole moments, considering {thirteen} small molecules (hydridoboron, hydrogen chloride, water, hydrogen sulfide, boron fluoride, carbon monoxide, dinitrogen, ethylene, formaldehyde, thioformaldehyde, nitroxyl, {fluorocarbene}, and silylidene). We systematically include corrections up to the quintuple (CCSDTQP) in the CC expansion and extrapolate to the complete basis set limit. When comparisons with experimental measurements are possible, that is, when a number of consistent experimental data can be found, theory typically provides values falling within the experimental error bar for the excited-state properties. Besides completing our previous studies focussed on transition energies (\textit{J.~Chem.~Theory Comput.} \textbf{14} (2018) 4360--4379, \textit{ibid.}~\textbf{15} (2019) 1939--1956, \textit{ibid.}~\textbf{16} (2020) 1711--1741, and \textit{ibid.}~\textbf{16} (2020) 3720--3736), this work also provides ultra-accurate dipoles and oscillator strengths that could be employed for future theoretical benchmarks.

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Context. Line shapes of the magnesium resonance lines in white dwarf spectra are determined by the properties of magnesium atoms and the structure of the white dwarf atmosphere. Through their blanketing effect, these lines have a dominant influence on the model structure and thus on the determination from the spectra of other physical parameters that describe the stellar atmosphere and elemental abundances.Aims. In continuation of previous work on Mg+He lines in the UV, we present theoretical profiles of the resonance line of neutral Mg perturbed by He at the extreme density conditions found in the cool largely transparent atmosphere of DZ white dwarfs.Methods. We accurately determined the broadening of Mg by He in a unified theory of collisional line profiles using ab initio calculations of MgHe potential energies and transition matrix elements among the singlet electronic states that are involved for the observable spectral lines.Results. We computed the shapes and line parameters of the Mg lines and studied their dependence on helium densities and temperatures. We present results over the full range of temperatures from 4000 to 12 000 K needed for input to stellar spectra models. Atmosphere models were constructed for a range of effective temperatures and surface gravities typical for cool DZ white dwarfs. We present synthetic spectra tracing the behavior of the Mg resonance line profiles under the low temperatures and high gas pressures prevalent in these atmospheres.Conclusions. The determination of accurate opacity data of magnesium resonance lines together with an improved atmosphere model code lead to a good fit of cool DZ white dwarf stars. The broadening of spectral lines by helium needs to be understood to accurately determine the H/He and Mg/He abundance ratio in DZ white dwarf atmospheres. We emphasize that no free potential parameters or ad hoc adjustments were used to calculate the line profiles.

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 (\ie, 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.

Following the recent work of Eriksen et al. [arXiv:2008.02678], we report the performance of the \textit{Configuration Interaction using a Perturbative Selection made Iteratively} (CIPSI) method on the non-relativistic frozen-core correlation energy of the benzene molecule in the cc-pVDZ basis. Following our usual protocol, we obtain a correlation energy of $-863.4(5)$ m$E_h$ which agrees with the theoretical estimate of $-863$ m$E_h$ proposed by Eriksen et al. using an extensive array of highly-accurate new electronic structure methods.

The density matrix renormalization group in chemistry and molecular physics: Recent developments and new challenges The Journal of Chemical Physics 152, 040903 (2020); https://doi. ABSTRACT Taking as an example the simple CH 3 radical, this work demonstrates the cooperative character of the spin-polarization phenomenon of the closed-shell core in free radicals. Spin polarization of CH σ bonds is not additive here, as spin polarization of one bond enhances that of the next bond. This cooperativity is demonstrated by a series of configuration interaction calculations converging to the full valence limit and is rationalized by analytic developments. The same phenomenon is shown to take place in those diradicals where spin polarization plays a major role, as illustrated in square planar carbo-cyclobutadiene C 12 H 4. The treatment of cooperativity represents a challenge for usual post-Hatree-Fock methods. Published under license by AIP Publishing. https://doi.

Atrazine Atomic and molecular structure and dynamics Anderson mechanism Contact electron density Pesticide CIPSI Spin-orbit interactions Perturbation theory Chemical Physics Cooperative effect BIOMOLECULAR HOMOCHIRALITY 3315Fm Relativistic corrections Chiral oxorhenium Petascale Car-Parrinello molecular dynamics Brown dwarfs Quantum Chemistry Argile Excited states CP violation Analytic gradient Conditions aux limites périodiques AB-INITIO Dispersion coefficients Configuration interactions 3115ag Chemical concepts Atrazine-cations complexes 3115ae Atomic processes Béryllium Complex plane Calcul ab initio CHEMICAL-SHIFTS Valence bond Molecular properties Circular dichroism Basis sets Charge conjugation symmetry ALGORITHM Pesticides Metabolites Clustering Molecular modeling Environmental fate Partial least squares 3115vj Parallel speedup Parity violation 3115aj Density functional theory Acrolein CP Violation Wave functions Time-dependent density-functional theory Aimantation Single-core optimization Carbon Nanotubes Argon Chiral transition metal complexes Electron electric moment Abiotic degradation AROMATIC-MOLECULES 3470+e Atom Configuration Interaction 3115am Coupled cluster calculations 3115vn Corrélation et relativité Polarizabilities Benzene 3115bw Corrélation électronique Line formation Chimie quantique Azide Anion Quantum Monte Carlo Cluster coupling Clay mineral Contact density Correlation and relativity Relativistic quantum chemistry Charge transfer state Xenon Chemical-Bonds Chiral halogenomethanes Configuration interaction CLUSTERS Range separation Beyond Standard Model Atomic data BENZENE MOLECULE Hyperfine structure Benchmarks Diatomic molecules Large systems Boys AB-INITIO CALCULATION Biodegradation Ab initio calculation Ground states Electron correlation COMPUTATION