General Meeting of the GDR REST

In this talk I will present recent measurements and theoretical calculations on a well known material: the hexagonal boron nitride. In particular I will discuss high energy excitations and failure and success of the GW+BSE approach. Finally I will discuss the excitons behaviour with pressure in hexBN.

Linear response is often used to compute the dynamical response properties of solids and molecules. However, its numerical approximation is sometimes problematic. In particular, ionization processes require a representation of the continuous spectrum of the Hamiltonian, which collapses to a discrete spectrum when truncated in a finite box. I will describe recent efforts aimed at a rigorous understanding of these effects, as well as the development of more efficient methods. This is joint work with M-S. Dupuy.

Optical and fundamental energy gaps can be described, in principle exactly, within the same time-independent density-functional ensemble formalism [1]. In density-functional theory (DFT) for ensembles, modeling the ensemble weight dependence of the exchange-correlation (xc) density functional is a central issue. In this presentation, I will show how weight dependencies relate to xc derivative discontinuities [2]. If time permits, I will discuss various strategies for incorporating weight dependencies into density functional approximations, with a particular focus on recently revealed ensemble density-driven correlations [3,4], which do not exist in standard (pure-state) DFT.

[1] B. Senjean and E. Fromager, Phys. Rev. A 98, 022513 (2018).
[2] M. J. P. Hodgson, J. Wetherell, and E. Fromager, Phys. Rev. A 103, 012806 (2021).
[3] T. Gould and S. Pittalis, Phys. Rev. Lett. 123, 016401 (2019).
[4] E. Fromager, Phys. Rev. Lett. 124, 243001 (2020).

The helium atom and the hydrogen molecule are almost unique many-body (though two-body only) real systems where we own an exact solution of the Schroedinger equation. Although not in a analytical closed-form, the Hylleraas-like solutions for He and H$_2$ are exact in a numerical analysis mathematical sense, that is, we can establish a rigorous absolute, non-statistical, error bar which can be reduced to any accuracy (today up to 40 digits octuple precision). These solutions are an ideal benchmark for approximate many-body approaches which, so far, have been checked only against the experiment or exact solutions of idealized models. Indeed, the former is affected by many other masking effects (nuclear motion, electron-phonon, relativistic and QED radiative corrections, etc.), while the latter represent a benchmark against over-simplified situations (discretization, non-Coulomb shortened-range interaction, etc.) more or less far from real world.
We will show a comparison of Hylleraas He and James & Coolidge H$_2$ exact solutions against several many-body theories from condensed-matter and nuclear physics, as well as quantum chemistry: configuration interaction (CI), quantum Monte Carlo (QMC, both variational VMC and diffusion DMC), density-functional theory (DFT) and time-dependent DFT (TDDFT), direct RPA and full RPA with exchange (aka time-dependent Hartree-Fock, TDHF), Bethe-Salpeter equation (BSE) on top of the GW approximation, and finally a renormalized RPA (r-RPA) in the framework of the self-consistent RPA (SCRPA) originally developed in nuclear physics.
The comparison will be done on both ground and excited states. On the ground state we will present the results by the trace Thouless formula which coincides to the ACFDT adiabatic-connection fluctuation-dissipation theorem) formula in direct RPA, but it appears better founded beyond RPA.

Using powerful tensor network methods, we are able to compute absorption spectra for real molecules in a solvent environment. The calculated spectra are exact within a description based on a linear vibronic coupling Hamiltonian and may be computed at any temperature at a modest computational cost thanks to a recently introduced technique which allows one to obtain finite temperature open system dynamics from a single, pure state wave-function simulation at 0K. These methods are applied to the investigation of the absorption spectrum of the widely used dye and medication, Methylene blue (MB). By performing simulations on models parameterized using Density Function Theory (DFT) and molecular dynamics (MD) we are able to demonstrate that the characteristic shoulder in the MB absorption spectrum arises from the presence of a previously overlooked dark excited state that is vibronically coupled to the lowest excited singlet state.
Furthermore, we find that the shape of this shoulder is highly sensitive to the presence of correlations between the environment-induced fluctuations of the bright and dark state energies, and that by introducing such correlations into our model we are able to obtain an excellent fit with experimental data. As well as providing microscopic insight into the spectral signatures of optical dark states in organic systems, this work also provides a means of comparing different levels of theory in DFT and MD in terms of their ability to reproduce experimentally observed spectra.

The difficulty of understanding strongly correlated matter is often related to the absence of a natural small parameter providing a starting point for a perturbative treatment. Large-N approaches provide a prominent theoretical tool, which seeks to address
this issue for fermionic theories by introducing a generalised theory with many fermion flavours, that then becomes amenable to perturbative treatments. 

We demonstrate how to implement this concept in conjunction with a diagrammatic Monte-Carlo approach, using a many fermion-flavour generalisation of the unitary Fermi gas [1] for illustration. This system provides an ideal test case thanks to existing diagrammatic Monte-Carlo techniques developed in [2,3], which yield a numerically exact solution thanks to explicit resummation of the high-order asymptotics [4]. 
We present results on the extended large-N generalisation of the theory [5,6], and demonstrate how to apply diagMC in this setting. Using the resummation technique developed by Rossi et al [4], we show that the convergence radius in the Borel plane is enlarged as a function of fermion flavours, thus facilitating the convergence of the series in the vicinity of the transition into the superfluid phase. We argue that the combination of large-N field theory techniques with high-order numerical resummations opens up a new avenue for investigations of strongly interacting systems more generally.
[1] W. Zwerger, Springer (2012).
[2] K. v.Houcke et al., Nat Phys 8, 366 (2012).
[3] K. v.Houcke et al., PRB 99, 035140 (2019).
[4] R. Rossi, et al., PRL 121, 130405 (2018).
[5] M.Y. Veillette, et al., PRA 75, 043614 (2007).
[6] P. Nikolic and S. Sachdev, PRA 75, 033608 (2007).

In the Kohn-Sham formulation of density functional theory (DFT) [1], the ground-state density of interacting electrons can be obtained from a fictitious system of independent particles in an effective potential. Although DFT is in principle exact the effective potential contains an unknown quantity called the exchange-correlation (xc) potential. In this talk we propose a new approximation to the xc potential using a general approach called "Connector Theory" (COT) [2]. This approach is a prescription of how to use data from models to calculate quantities in materials. COT is in principle exact but in practice approximations are needed to make it useful. After introducing the general scheme of this approach, we explain how to use our approximate xc potential in the self-consistent Kohn-Sham loop to calculate the electronic density. Finally, by comparing the resulting electron density of silicon with benchmark Quantum Monte Carlo results [3], we find a very good agreement.
[1] P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 (1964).
[2] M. Vanzini, A. Aouina, M. Panholzer, M. Gatti, and L. Reining, arXiv:1903.07930
[3] S. Chen, M. Motta, F. Ma and S. Zhang, Phys. Rev. B 103, 075138 (2021).

In order to describe spectroscopic properties of correlated systems, the Green's functions (GF) formalism appears to be one of the most attractive. In that line, several methods has been develloped such has Dynamical Mean Field Theory formalism [1] , where the local GF is extracted from an Anderson Impurity Model wich shares the same local GF as the original system. However, this mapping remains highly non trivial and numerically expensive. To obtain the local GF with a cheap numerical cost and in a conceptually efficient way, the Anderson Impurity Model can be reduced into a smaller one [2,3] or the original system can be divided into a explicit embedded systems [4,5].
In this communication, both approches are used efficiently to extract the local GF as a functional of the 1-body reduced density matrix (1RDM). More precisely, the local GF is extracted from an embedded correlated system by applying the unitary Householder transformation to the density matrix. The self-consistent mapping between the original and the embedded system is ensure via an explicit hybridization function. However, the representability domain of the 1RDM obtained from the GF (itself extracted from the embedded system) is restricted and will be discuss within the Self-energy Functional Theory [6] framework.
[1] : A. Georges and G. Kotliar, Hubbard Model in Infinite Dimensions, Phys. Rev. B 45, 6479 (1992).
[2] : M. Caffarel and W. Krauth, Exact Diagonalization Approach to Correlated Fermions in Infinite Dimensions: Mott Transition and Superconductivity, Phys. Rev. Lett. 72, 1545 (1994).
[3] : Potthoff M. Dynamical Variational Principles for Strongly Correlated Electron Systems. In: Kramer B. (eds) Advances in Solid State Physics. Advances in Solid State Physics, vol 45. Springer, Berlin
[4] : E. Fertitta and G. H. Booth, Rigorous Wave Function Embedding with Dynamical Fluctuations, Phys. Rev. B 98, 235132 (2018).
[5] : A. A. Rusakov, S. Iskakov, L. N. Tran, and D. Zgid, Self-Energy Embedding Theory (SEET) for Periodic Systems, J. Chem. Theory Comput. 15, 229 (2019).
[6] : M. Potthoff, Self-Energy-Functional Approach to Systems of Correlated Electrons, The European Physical Journal B - Condensed Matter 32, 429 (2003).

The kinetic energy loss of an impinging ion in matters per unit of penetration length is named the stopping power. Calculating this quantity has been a long-standing goal for physicists. Even the famous Lindhard dielectric function of the homogeneous electron gas [1] was derived in pursuit of this purpose.
Modern evaluations of the stopping power rely on time-dependent density-functional theory (TDDFT). When implemented in real-time (RT), these calculations are able to capture intriguing effects that go beyond the linear-response approximation (LR).
Our presentation will focus on two main subjects:
First, we have investigated the Barkas effect [2] in insulating materials. This beyond-linear-response effect states that the stopping power of antiprotons (H-) is always lower than that of protons (H+). Using RT-TDDFT calculations, we have explored the low projectile kinetic energy region (below 2 keV) in a standard insulating material, LiF. Surprisingly, we have observed an inverse Barkas effect: the stopping power of antiprotons becomes larger than the one of protons. To investigate further, we have performed both RT and LR calculations on isolated Li+ and F- targets. We see that the binding between F- and its p electrons is weakened in the presence of an antiproton.
Second, we will talk about our progress in stopping power calculations with moving localized basis for heavier ion projectiles that have core electrons moving along their nucleus. In this case, the time-dependent Kohn-Sham equations contain an extra term [3] analogous to the Pulay forces [4]. We have implemented this term in our code MOLGW [5] and have performed calculations for atomic collisions and gas-phase targets. Our ultimate goal will be to describe heavier ions such as transition metal cations in crystalline targets.

REFERENCES:
[1] J. Lindhard, Danske Matematisk-fysiske Meddeleiser, 28, 1 (1954).
[2] W. Barkas, J. Dyer and H. Heckman, Phys. Rev. Lett., 11, 26-28 (1963)
[3] T. Kunnert and R. Schmidt, Eur. Phys. J. D, 25, 15-24 (2003)
[4] P. Pulay, Molecular Physics, 17, 2, 197-204 (1969)
[5] www.molgw.org

To face the continuing demand for large density and energy efficient data storage devices, the possibilities of manipulating the magnetization without relying on the generation of a magnetic field are widely being investigated[1]. One of the most promising methods is the use of femtosecond light pulses. Such a light can trigger the so–called all–optical helicity–dependent switching (AO–HDS), which presence has been evidenced in many different types of magnetic thin films [2]. AO–HDS allows for the control of the magnetization state by solely relying on circularly left (σ + ) or right (σ − ) polarized light pulses of a few tens of femtoseconds, and, as a consequence, is a prime candidate for the disruption of data storage technologies.
While the origin of the AO–HDS is still debated, it is usually assumed that the dominating contribution to this effect comes from the inverse Faraday effect (IFE)[3, 4], designating the generation of a magnetization proportional to the intensity of the circularly polarized light. Another effect, occurring simultaneously to the IFE in dissipative materials is the magnetization induced during light absorption[5] (MILA), whose magnitude is proportional to the fluence of the light.
Recently, real–time time–dependent density functional theory (RT–TDDFT) has been successfully applied to describe the linearly polarized laser induced demagnetization[6]. In the present work[7] we investigate the dynamics of the magnetization density of Ni, Co and Fe under the influence of optical and XUV circularly polarized pump pulses using RT–TDDFT. We find that, in both cases, the induced spin–dynamics is helicity-dependent, and, that this helicity–dependence is larger when using a XUV light. The origin of this helicity-dependence is closely assessed by examining the spin–resolved time–dependent density of states. Then, we separate contribution of the IFE and the MILA in the helicity–dependent part of the magnetization dynamics and show that the latter has a greater impact, especially in the XUV regime and when the light pulse vanishes.
This work hints at the yet experimentally unexplored territory of the XUV light-induced helicity-dependent dynamics, which, according to our prediction, could magnify the helicity-dependent dynamics already exhibited in the optical regime.

[1] D. Sander, S. O. Valenzuela, D. Makarov, C. H. Marrows, E. E. Fullerton, P. Fischer, J. McCord, P. Vavassori, S. Mangin, P. Pirro, B. Hillebrands, A. D. Kent, T. Jungwirth, O. Gutfleisch, C. G. Kim, and A. Berger, “The 2017 Magnetism Roadmap,” Journal of Physics D: Applied Physics, vol. 50, p. 363001, sep 2017.
[2] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and T. Rasing, “All-optical magnetic recording with circularly polarized light,” Physical Review Letters, vol. 99, p. 47601, jul 2007.
[3] L. P. Pitaevskii, “Electric Forces in a Transparent Dispersive Medium,” J. Exptl. Theoret. Phys. (U.S.S.R.), vol. 12, no. 5, p. 1450, 1961.
[4] M. Berritta, R. Mondal, K. Carva, and P. M. Oppeneer, “Ab Initio Theory of Coherent Laser-Induced Magnetization in Metals,” Physical Review Letters, vol. 117, p. 137203, sep 2016.
[5] P. Scheid, G. Malinowski, S. Mangin, and S. Lebègue, “Ab initio theory of magnetization induced by light absorption in ferromagnets,” Phys. Rev. B, vol. 100, no. 21, p. 214402, 2019.
[6] K. Krieger, J. K. Dewhurst, P. Elliott, S. Sharma, and E. K. Gross, “Laser-Induced Demagnetization at Ultrashort Time Scales: Predictions of TDDFT,” Journal of Chemical Theory and Computation, vol. 11, no. 10, pp. 4870–4874, 2015.
[7] P. Scheid, S. Sharma, G. Malinowski, S. Mangin, and S. Lebègue, “Ab Initio Study of Helicity-Dependent Light-Induced Demagnetization: From the Optical Regime to the Extreme Ultraviolet Regime.,” Nano letters, vol. 21, pp. 1943–1947, mar 2021.

During this contribution we would like to introduce a new linear algebraic theorem inspired from inequalities in matrix perturbation theory. Combining this theorem with some known perturbation bounds for matrix eigenvalues we will provide the exact boundary value for the amount of charge transferred during a light-induced molecular electronic transition. Such matrix inequalities are derived in the case where orbital relaxation is accounted for in the representation of an electronic transition through the addition of the so-called Z-vector to the unrelaxed TDDFT one-body reduced difference density matrix. Few interesting matrix equalities in the case where the detachment/attachment density-matrix machinery is applied to unrelaxed/relaxed difference density matrix instead of ground/excited-state difference density matrix will be introduced in order to measure and visualize the global effects of orbital relaxation on the electronic-structure reorganization occurring during the electronic transition.
Besides, we will reveal the ambiguity of the picture of a molecular electronic transition considering the canonical tools that are still currently used for visualizing a molecular electronic transition and quantifying its nature. Without the disambiguation we suggest to introduce, and considering the possibility to imply auxiliary many-body wavefunctions as a possible picture, the representation of an unrelaxed TDDFT electronic transition would be either incomplete, arbitrary, or equivocal.

[1] A. Krylov, From orbitals to observables and back, JCP 153, 080901 (2020)
[2] S. Bäppler, F. Plasser, M. Wormit, A. Dreuw, Exciton analysis of many-body wave functions: Bridging the gap between the quasiparticle and molecular orbital pictures, PRA 90, 052521 (2014)
[3] T. Etienne, A comprehensive, self-contained derivation of the one-body density matrices from single-reference excited-state calculation methods using the equation-of-motion formalism, IJQC 120, e26110 (2020)
[4] T. Etienne, M. Pastore, Charge separation: From the topology of molecular electronic transitions to the dye/semiconductor interfacial energetics and kinetics, in Dye-sensitized solar cells: Mathematical modeling, and materials design and optimization, Masoud Soroush and Kenneth K.S. Lau Eds., Elsevier Academic Press, pp. 121-170 (2019)
[5] F. Plasser, M. Wormit, A. Dreuw, New tools for the systematic analysis and visualization of electronic excitations. I. Formalism, JCP 141, 024106 (2016)

Dichroism is observed when the absorption spectrum of a material is influenced by the polarisation of incident photons. Natural circular dichroism is expressed as the difference in the absorption spectra of right and left circularly polarised X-rays in the absence of any magnetization.
Natural circular dichroism, first observed in 1895 by Aimé Cotton, is one of the most powerful tools for obtaining stereochemical information, and is the only method outside of X-ray crystallography capable of revealing the absolute configuration of a chiral system. Circular dichroism measurements have first been developed in the UV-Visible range and have recently been extended to X-ray absorption spectroscopy (XAS, where it received the name of X-ray Natural Circular Dichroism, XNCD). Up to now, XNCD has only been observed in crystals. Due to the intrinsic chemical selectivity of XAS, XNCD mixes information from the local environment of the absorbing atom to the symmetry of the crystal itself. XNCD can only be observed for crystals for which the point of the space group belongs to a family of 13 point groups. Although the theoretical basis seems rather straightforward, there are very few examples of the calculation of XNCD spectra and so far there is not much understanding on how the observed shape of XNCD signals relate to the local point group of the absorbing atom or the point group of the space group of the crystal. Following previous work by Nadejda Bouldi [1], we have performed a theoretical investigation of the XNCD signals in the chiral crystal of α-quartz.
One of the structures for SiO 2 is α-quartz that crystalizes in one of the two chiral space groups P3 1 21 and P3 2 21. These two space groups are compatible with the observation of XNCD. By the way, α-quartz has no magnetic ion so that its net magnetization is zero and one cannot expect to detect any XMCD (X-ray Magnetic Circular Dichroism) when recording XNCD. We have calculated XNCD and the angular dependence of the signal using the code Quantum Espresso, that is based on Density Functional Theory (DFT). Quantum Espresso uses a plane wave basis set and pseudo-potentials. For the λ and ∆ enantiomers, we shall present the XNCD signals at both Si and O K-edges and their angular dependence. The ultimate target of the calculation is to make connection between the XNCD signals and some operator, the pseudo-deviator, that is related to the intensity of the integrated XNCD signal.

[1] N. Bouldi, N. J. Vollmers, C. G. Delpy-Laplanche, Y. Joly, A. Juhin, Ph. Sainctavit, Ch. Brouder, M.
Calandra, L. Paulatto, F. Mauri, and U. Gerstmann. X-ray magnetic and natural circular dichroism from first principles: calculation of K- and L1- edge spectra. Physical Review B 96, 085123 (2017)

Metal-organic frameworks (MOFs) have been attracting much attention in the past 20 years as possible candidate materials for a variety of applications, such as in gas capture and separation, drug delivery, sensors and optoelectronics.
Recently, photoresponsive MOFs have been proposed as a potentially efficient technology for gas capture and release as light, possibly in the visible range, rather than temperature, can be used to change the gas adsorption properties of the material. Recent experimental efforts have demonstrated that this can be achieved by functionalizing MOFs using molecular photoswitches such as azobenzenes [1]. However, due to the lack of computational studies on the subject, the nature of the excitations in the MOF and their similarity with the molecular case have not been established so far and yet, this would have strong implications on the efficiency of the process.
In our work we have address this point by computing the electronic structure and the optical absorption spectra of the MOF in trans and cis configuration using the BSE/GW method. In order to predict the BSE spectra with good accuracy, an orbital-tuning approach will be employed to select the exchange and correlation functional for the initial DFT calculations [2]. Finally, periodic and non-periodic BSE and GW calculations within a QM/MM scheme will be performed and compared to assess the the validity of fragment models [3]: if justified it will allow for a more efficient design of the optimal MOF for energy-efficient gas capture-and-release.

[1] Park et al, Reversible alteration of CO 2 adsorption upon photochemical or thermal treatment in a MOF. J. Am. Chem. Soc. 2012, 134, 99–102, DOI: https://doi.org/10.1021/ja209197;
[2] Kshirsagar, D’Avino, Blase, Li, Poloni, Accurate Prediction of the S 1 Excitation Energy in Solvated Azobenzene Derivatives via Embedded Orbital-Tuned Bethe-Salpeter Calculations. J. Chem. Theory Comput. 2020, 16, 2021– 2027, DOI: https:doi.org/10.1021/acs.jctc.9b01257;
[3] Kshirsagar, Attaccalite, Blase, Li, Poloni, Bethe–Salpeter Study of the Optical Absorption of trans and cis Azobenzene-Functionalized Metal–Organic Frameworks Using Molecular and Periodic Models J. Phys. Chem. C 2021, DOI: https://doi.org/10.1021/acs.jpcc.1c00367

The standard ab initio method for solid state physics, the Density Functional Theory (DFT), fails to give a correct description of the density of states and structural parameters of strongly correlated systems. The DFT+U improves this description by explicitly applying electronic interaction inside specific orbitals.
This method is extensively used in f and d orbitals of transition metals, lanthanides and actinides. However, the correlation effect occurring in the p orbitals have been shown to improve the density of states and structural parameters of simple oxides. [1-3] In this paper, we use our generalized cRPA implementation (as described in [4] and based on [5]), to calculate the screened interaction on both metal (d or f ) and oxygen (p) orbitals . We also discuss the influence of those interaction on density of state and structural parameters of both charge transfer and Mott-Hubbard insulators.

1 I. A. Nekrasov, M. A. Korotin, and V. I. Anisimov, “Coulomb interaction in oxygen p-shell in LDA+U method and its influence on calculated spectral and magnetic properties of transition metal oxides”, arXiv:cond-mat/0009107 (2000).
2 L. A. Agapito, S. Curtarolo, and M. Buongiorno Nardelli, “Reformulation of DFT+U as a Pseudohybrid Hubbard Density Functional for Accelerated Materials Discovery”, Phys. Rev. X 5, 011006 (2015).
3 O. K. Orhan and D. D. O’Regan, “First-principles Hubbard U and Hund’s J corrected approximate density functional theory predicts an accurate fundamental gap in rutile and anatase TiO 2 ”, Phys. Rev. B 101, 245137 (2020).
4 J.-B. Morée, R. Outerovitch, and B. Amadon, “First-principles calculation of the Coulomb interaction parameters U and J for actinide dioxides”, Phys. Rev. B 103, 045113 (2021).
5 P. Seth, P. Hansmann, A. van Roekeghem, L. Vaugier, and S. Biermann, “Towards a First-Principles Determination of Effective Coulomb Interactions in Correlated Electron Materials: Role of Intershell Interactions”, Phys. Rev. Lett. 119, 056401 (2017).

Chromia (Cr2O3) is a typical Mott Hubbard anti-ferromagnetic insulator in the class of transition metal oxides. Cr2O3 is an ideal material to demonstrate ultra-fast control of demagnetization at higher speed [1].
Ultra-fast control on the magnetic state could have the strong impact on magnetic recording technology. Second harmonic generation (SHG) is a process where two photons are absorbed by a material, and a photon of twice the energy of the incoming photons is emitted. This spectroscopy is used to study the optical properties of materials because it reveals additional information, compared with linear optical spectroscopies. Due to dipole selection rules, SHG is forbidden in centrosymmetric materials, and it is possible to obtain a structural and electronic characterization for these systems. However, the absence of time-inversion and space symmetry in antiferromagnetic materials leads to new contributions in the second harmonic generation, thus revealing the arrangement of spins in the solid. SHG becomes a powerful tool to study of ultra-fast demagnetization processes.
There are few satisfactory theoretical descriptions for SHG in magnetic materials, since spin- polarization, electron-electron interactions, and local field effect must be treated on the same footing. The project is to calculate the Second order response function of Cr2O3 using Time-Dependent Density Functional Theory including spin-polarization. The linear response of Cr2O3 using different approximations (Independent Particle approximation (IPA), Random Phase approximation (RPA), and alpha kernel is calculated in the TDDFT framework. The value of band gap for the Cr2O3 and the scissor shift is calculated using GW approximation. The BSE calculations is performed using scissor shift and BSE spectra is obtained. The strongly bound excitonic peak has been observed in the BSE spectra. The presence of this peak is important, as in the experiment performed [1], the SHG signal has been measured in this range of frequency. The transition which are responsible for the excitonic peak has been studied. The excitonic peak has been described using the Wannier model. The BSE spectra using the spin-polarization has been calculated. The second-order response function including spin-polarization is calculated using Random Phase approximations (RPA). We concluded that second-order response functions of Cr2O3 including spin-polarization is non-zero.

[1] V. G. Sala et al., “Resonant optical control of the structural distortions that drive ultrafast demagnetization in Cr2 O3,” Phys. Rev. B, vol. 94, no. 1, Jul. 2016.

Understanding the mechanism of interaction between amino acids and peptides with surfaces opens new perspectives. Adsorption of DNA molecules on semiconducting surfaces is evoked for the design of bio-sensors or the production of bio-materials. The possibility to functionalize surfaces with bio-molecules, to create organized structures up to nanometers’ distances depends on our capability to understand precisely the mechanisms which govern the deposition of molecular films onto different kinds of surfaces. For these reasons, this kind of systems have been intensively studied during last decades [1][2].
We present in this contribution the results obtained for the Si(001) surface functionalized with two different molecules, the Thymine and the Uracil. The electronic structure has been obtained with a DFT approach, using Abinit [3]. Using DP [4] and 2Light codes [5] [6], which allows the calculation of the optical response of materials in the TDDFT framework, we computed several surface-related linear and nonlinear spectroscopic quantities.

[1] K. Seino, W.G. Schmidt, F. Bechstedt, Phys. Rev. B 69, 245309 (2004)
[2] E. Molteni, G. Cappellini, G. Onida, and G. Fratesi, Phys. Rev. B 95, 075437 (2017)
[3] X. Gonze et al., Comput. Phys. Commun. 205, 106-131 (2016)
[4] http://dp-code.org/
[5] E. Luppi, V. Véniard, Phys. Rev. B 82, 235201 (2010)
[6] N. Tancogne-Dejean, C. Giorgetti, V. Véniard, Phys. Rev. B 94, 125301 (2016)

Using a one-dimensional (1D) Helium-like atom with a δ-function interaction we aim at providing a deeper understanding of the recently introduced basis-set correction method to wave-function theory calculations in the general context of atomic and molecular physics [1]. Using a δ-function interaction permits to reproduce the problematic electron-electron cusp which is the cause of slow convergence with basis-set size of the energy calculated with correlated wave functions. As a first step toward defining basis-set correction for this model, we examine the local-density approximation (LDA) based on a finite 1D uniform electron gas (UEG) with two electrons.
We calculate the correlation energy density of this UEG and parametrize it and compare with the corresponding infinite 1D UEG [2].

[1] E. Giner, B. Pradines, A. Ferté, R. Assaraf, A. Savin and J. Toulouse, J. Chem. Phys. 149, 194301 (2018).
[2] R. J. Magyar and K. Burke, Phys. Rev. A 70, 032508 (2004).