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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.

We describe our efforts of the past few years to create a large set of more than 500 highly-accurate vertical excitation energies of various natures ($\pi \to \pi^*$, $n \to \pi^*$, double excitation, Rydberg, singlet, doublet, triplet, etc) in small- and medium-sized molecules. These values have been obtained using an incremental strategy which consists in combining high-order coupled cluster and selected configuration interaction calculations using increasingly large diffuse basis sets in order to reach high accuracy. One of the key aspect of the so-called QUEST database of vertical excitations is that it does not rely on any experimental values, avoiding potential biases inherently linked to experiments and facilitating theoretical cross comparisons. Following this composite protocol, we have been able to produce theoretical best estimate (TBEs) with the aug-cc-pVTZ basis set for each of these transitions, as well as basis set corrected TBEs (i.e., near the complete basis set limit) for some of them. The TBEs/aug-cc-pVTZ have been employed to benchmark a large number of (lower-order) wave function methods such as CIS(D), ADC(2), CC2, STEOM-CCSD, CCSD, CCSDR(3), CCSDT-3, ADC(3), CC3, NEVPT2, and others (including spin-scaled variants). In order to gather the huge amount of data produced during the QUEST project, we have created a website [https://lcpq.github.io/QUESTDB_website] where one can easily test and compare the accuracy of a given method with respect to various variables such as the molecule size or its family, the nature of the excited states, the type of basis set, etc. We hope that the present review will provide a useful summary of our effort so far and foster new developments around excited-state methods.

Using a set of oscillator strengths and excited-state dipole moments of near full configuration interaction (FCI) quality determined for small compounds, we benchmark the performances of several single-reference wave function methods (CC2, CCSD, CC3, CCSDT, ADC(2), and ADC(3/2)) and time-dependent density-functional theory (TD-DFT) with various functionals (B3LYP, PBE0, M06-2X, CAM-B3LYP, and $\omega$B97X-D). We consider the impact of various gauges (length, velocity, and mixed) and formalisms: equation of motion (EOM) \emph{vs} linear response (LR), relaxed \emph{vs} unrelaxed orbitals, etc. Beyond the expected accuracy improvements and a neat decrease of formalism sensitivy when using higher-order wave function methods, the present contribution shows that, for both ADC(2) and CC2, the choice of gauge impacts more significantly the magnitude of the oscillator strengths than the choice of formalism, and that CCSD yields a notable improvement on this transition property as compared to CC2. For the excited-state dipole moments, switching on orbital relaxation appreciably improves the accuracy of both ADC(2) and CC2, but has a rather small effect at the CCSD level. Going from ground to excited states, the typical errors on dipole moments for a given method tend to roughly triple. Interestingly, the ADC(3/2) oscillator strengths and dipoles are significantly more accurate than their ADC(2) counterparts, whereas the two models do deliver rather similar absolute errors for transition energies. Concerning TD-DFT, one finds: i) a rather negligible impact of the gauge on oscillator strengths for all tested functionals (except for M06-2X); ii) deviations of ca.~0.10 D on ground-state dipoles for all functionals; iii) the better overall performance of CAM-B3LYP for the two considered excited-state properties.

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

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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.

We calculate interaction constants for the contributions from \PT-odd scalar-pseudoscalar and tensor-pseudotensor operators to the electric dipole moment of ${}^{129}$Xe, for the first time in case of the former, using relativistic many-body theory including the effects of dynamical electron correlations. These interaction constants are necessary ingredients to relating the corresponding measurements to fundamental parameters in models of physics beyond the Standard Model. We obtain $\alpha_{C_S} = \left( 0.71 \pm 0.18 \right) [10^{-23}\, e~\text{cm}]$ and $\alpha_{C_T}= \left( 0.520 \pm 0.049 \right) [10^{-20}\, \left<\Sigma\right>_{\text{Xe}}\, e~\text{cm}]$, respectively. We apply our results to test a phenomenological relation between the two quantities, commonly used in the literature, and discuss their present and future phenomenological impact.

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