Excited-State Electronic Structure of Molecules Using **Many**-**Body** Green's Functions: Quasiparticles and Electron-Hole Excitations with VOTCA-XTP

**many**-

**body**Green’s functions theory in the GW approximation with the Bethe–Salpeter Equation (BSE). Expand abstract.

**many**-

**body**Green’s functions theory in the GW approximation with the Bethe–Salpeter Equation (BSE). This work provides a summary of the underlying theory and discusses details of its implementation based on Gaussian orbitals, including, i.a., resolution-of-identity techniques, different approaches to the frequency integration of the self-energy or acceleration by offloading compute-intensive matrix operations using GPUs in a hybrid OpenMP/Cuda scheme. A distinctive feature of VOTCA-XTP is the capability to couple the calculation of electronic excitations to a classical polarizable environment on atomistic level in a coupled quantum- and molecular-mechanics (QM/MM) scheme, where a complex morphology can be imported from Molecular Dynamics simulations. The capabilities and limitations of the GW -BSE implementation are illustrated with two examples. First, we study the dependence of optically active electron-hole excitations in a series of diketopyrrolopyrrole-based oligomers on molecular-architecture modifications and the number of repeat units. Second, we use the GW -BSE/MM setup to investigate the effect of polarization on localized and intermolecular charge-transfer excited states in morphologies of low-donor content rubrene-fullerene mixtures. These showcases demonstrate that our implementation currently allows to treat systems with up to 2500 basis functions on regular shared-memory workstations, providing accurate descriptions of quasiparticle and coupled electron-hole excited states of various character on an equal footing.

10/10 relevant

chemRxiv

Active Learning of **Many**-**Body** Configuration Space: Application to the Cs+–water MB-nrg Potential Energy Function as a Case Study

**many**-

**body**PEFs, with chemical and spectroscopic accuracy, for molecular simulations from the gas to condensed phase. Expand abstract.

**many**-

**body**molecular models poses a challenge to current data-driven approaches to molecular simulations. Here, we introduce an active learning (AL) framework for generating training sets corresponding to individual

**many**-

**body**contributions to the energy of a N-

**body**system, which are required for the development of MB-nrg potential energy functions (PEFs). Our AL framework is based on uncertainty and error estimation, and uses Gaussian process regression (GPR) to identify the most relevant configurations that are needed for an accurate representation of the energy landscape of the molecular system under exam. Taking the Cs+–water system as a case study, we demonstrate that the application of our AL framework results in significantly smaller training sets than previously used in the development of the original MB-nrg PEF, without loss of accuracy. Considering the computational cost associated with high-level electronic structure calculations for training set configurations, our AL framework is particularly well-suited to the development of

**many**-

**body**PEFs, with chemical and spectroscopic accuracy, for molecular simulations from the gas to condensed phase.

10/10 relevant

chemRxiv

On the Nature of Alkali Ion−Water Interactions: Insights from **Many**-**Body** Representations and Density Functional Theory. II.

**many**-

**body**potential energy functions, and exchange correlation functionals selected across the hierarchy of density functional theory approximations. Expand abstract.

**many**-

**body**potential energy functions, and exchange correlation functionals selected across the hierarchy of density functional theory approximations. Analysis of interaction energy decompositions indicates that close range interactions such as Pauli repulsion, charge transfer, and charge penetration must be captured in order to reproduce accurate interaction energies. In particular, it is found that simple classical polarizable models must be supplemented with dedicated terms which account for these close range interactions in order to achieve chemical accuracy across configuration space. It is also found that the XC functionals mostly differ from each other in their Pauli repulsion + Dispersion energies, and hence benefit from the inclusion of nonlocal terms such as Hartree-Fock exchange and dependence on the electronic kinetic energy density in order to reproduce the interactions that contribute to this term, namely Pauli repulsion and (intermediate-range) dispersion. As a continuation of the analysis performed in J. Chem. Theory Comput. 2019, 15, 2983, we make comparisons between findings for alkali ion−water interactions with those for halide−water interactions.

10/10 relevant

chemRxiv

Anomalous diffusion in particle-hole symmetric **many**-**body** localized
systems

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**body**localized (MBL) phase. Expand abstract.

**many**-

**body**localized (MBL) phase. We provide numerical evidence that it can be characterized by an algebraic propagation of both entanglement and charge, unlike in the conventional MBL case. We explain the mechanism of this anomalous diffusion through a formation of bound states, which coherently propagate via long-range resonances. By projecting onto the two-particle sector of the particle-hole symmetric model, we show that the formation and observed subdiffusive dynamics is a consequence of an interplay between symmetry and interactions.

10/10 relevant

arXiv

Derivation of the Landau-Pekar equations in a **many**-**body** mean-field limit

**many**bosonic particles weakly couple to the quantized phonon field. Expand abstract.

**many**bosonic particles weakly couple to the quantized phonon field. For large particle number and suitably small coupling, we show that the dynamics of the system is approximately described by the Landau-Pekar equations. These describe a Bose-Einstein condensate interacting with a classical polarization field, whose dynamics is effected by the condensate, i.e., the back-reaction of the phonons that are created by the particles during the time evolution is of leading order.

8/10 relevant

arXiv

Learning about learning by **many**-**body** systems

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**body**systems, from soap bubbles to suspensions to polymers, learn the drives that push them. Expand abstract.

**many**-

**body**systems, from soap bubbles to suspensions to polymers, learn the drives that push them. This learning has been characterized with thermodynamic properties, such as work dissipation and strain. We move beyond these macroscopic properties that were first defined for equilibrium contexts: We quantify statistical mechanical learning with machine learning. Our strategy relies on a parallel that we identify between representation learning and statistical mechanics in the presence of a drive. We apply this parallel to measure classification ability, memory capacity, discrimination ability, and novelty detection. Numerical simulations of a spin glass illustrate our technique. This toolkit exposes self-organization that eludes detection by thermodynamic measures. Our toolkit more reliably and more precisely identifies and quantifies learning by matter.

10/10 relevant

arXiv

Local Integrals of Motion for Topologically Ordered **Many**-**Body** Localized
Systems

**Many**-

**body**localized (MBL) systems are often described using their local integrals of motion, which, for spin systems, are assumed to be a local unitary transform of the set of on-site spin-z operators. Expand abstract.

**Many**-

**body**localized (MBL) systems are often described using their local integrals of motion, which, for spin systems, are assumed to be a local unitary transform of the set of on-site spin-z operators. We show that this assumption must be revised for topologically ordered MBL systems, and introduce a notion of local integrals of motion capturing such systems and beyond, in any spatial dimension. Using this framework, we demonstrate a number of features, including that MBL topological order, if present: (i) is the same for all eigenstates; (ii) is robust in character against any perturbation preserving MBL; (iii) implies that on topologically nontrivial manifolds a complete set of integrals of motion must include nonlocal ones in the form of local-unitary-dressed noncontractible Wilson loops. Our approach is well suited for tensor-network methods, and is expected to allow these to resolve highly-excited finite-size-split topological eigenspaces despite their overlap in energy. We illustrate our approach on the disordered Kitaev chain, the toric code, and the X-cube model.

10/10 relevant

arXiv

Finding universal structures in quantum **many**-**body** dynamics via
persistent homology

**many**-

**body**dynamics in terms of robust topological structures beyond standard field theoretic techniques. Expand abstract.

**many**-

**body**dynamics in terms of robust topological structures beyond standard field theoretic techniques.

10/10 relevant

arXiv

Many-**Body** Dephasing in a Trapped-Ion Quantum Simulator

**many**-

**body**system relaxes and dephases as a function of time is a fundamental question in thermodynamic and statistical physics. In this work, we observe and analyse the persistent temporal fluctuations after a quantum quench of a tunable long-range interacting transverse-field Ising Hamiltonian realized with a trapped-ion quantum simulator. We measure the temporal fluctuations in the average magnetization of a finite-size system of spin-$1/2$ particles and observe the experimental evidence for the theoretically predicted regime of

**many**-

**body**dephasing. We experiment in a regime where the properties of the system are closely related to the integrable Hamiltonian with global spin-spin coupling, which enables analytical predictions even for the long-time non-integrable dynamics. We find that the measured fluctuations are exponentially suppressed with increasing system size, consistent with theoretical predictions.

10/10 relevant

arXiv

Lieb-Robinson bounds and strongly continuous dynamics for a class of
**many**-**body** fermion systems in $\mathbb{R}^d$

**body**interactions for fermions in $\mathbb{R}^d$ and prove a Lieb-Robinson estimate for the dynamics of this class of

**many**-

**body**systems. As a step toward this result, we also prove a propagation bound of Lieb-Robinson type for Schr\"odinger operators. We apply the propagation bound to prove the existence of infinite-volume dynamics as a strongly continuous group of automorphisms on the CAR algebra.

10/10 relevant

arXiv