Seminars

The LCQS welcomed Benjamin Lasorne from Montpellier, ICGM and Nadia Ben Amor from Toulouse, LCPQ during the second week of July 2024.

The LCQS welcomed Giovanni Li Manni from Stuttgart Max Planck Institute during the first week of July 2024.

He will give a seminar Thursday, July 4th, 2024 on ‘Quantum Anamorphosis: A Novel Quantum Chemical Route for Predicting Magnetic Interactions in Polynuclear Transition Metal Clusters‘.

Abstract:

Understanding spin interactions in ground and excited states of polynuclear transition metal (PNTM) complexes, at the active site of biological and biomimetic catalysts, is of paramount importance for gaining control over their remarkable reactivity. The water splitting reaction at the oxygen evolving center of photosystem II, and the nitrogen fixation process catalyzed by FeS clusters in the nitrogenases
exemplify the versatility and complexity of such systems. Quantum chemical simulations of the spin structures and magnetic properties of PNTM clusters represent a major challenge for modern quantum chemistry, and multiconfigurational strategies are unavoidable. Cheap and almostaccurate strategies are an oxymoron!

Local emergent symmetries exist for exchange-coupled PNTM clusters that are revealed by a conceptual strategy based on simple unitary orbital transformations, together with cumulative spin adaptation. This strategy, named Quantum Anamorphosis in analogy to Anamorphosis in art, (a) greatly reduces the multi-reference character of strongly correlated electronic states, (b) allows for state-specific optimizations of excited states, and (c) offers a facile rationalization of the spin interactions across the magnetic sites. As for the homonym art, Quantum Anamorphosis guides the scientist toward a vantage point, in the orbital space, to obtain simple and easily recognizable electronic wave functions.

Quantum Anamorphosis is a multi-year project that has already been applied in the investigation of Fe₄^IIIS₄ complexes, Mn₃^IVO₄ clusters, the biomimetic Co₃^IIEr^IIIO₄ cubane, and in the study of lowdimensional (1D) nearest-neighbor Heisenberg Hamiltonian. The foundation of Quantum Anamorphosis and its application will be the focus of this talk.

The LCQS welcomed Emiel Koridon from Leiden Universiteit (The Netherlands)  during the first week of February 2024. If you want to learn more from Emiel’s work, here is his google scholar link.

He will give a seminar Tuesday, February 6th, 2024 on ‘Quantum Computational Chemistry for the Identification and Description of Conical Intersection Regions ‘.

Abstract:

The recent advances in near-term quantum devices give rise to a need for noise-robust algorithms that minimize device requirements. Working towards this goal from both the chemical as the quantum computational perspective we investigate the identification and description of conical intersections. Conical intersections are topologically protected crossings between the potential energy surfaces of a molecular Hamiltonian and play a pivotal role in chemical processes such as photoisomerization and non-radiative relaxation. They are characterized by a non-zero Berry phase, which is a topological invariant defined on a closed path in atomic coordinate space, taking the value π when the path encircles the intersection manifold. We show that for real molecular Hamiltonians, the Berry phase can be obtained by tracing a local optimum of a variational ansatz along the chosen path and estimating the overlap between the initial and final state with a control-free Hadamard test. Since the Berry phase can only take two discrete values (0 or π), our procedure succeeds even for a cumulative error bounded by a constant; this allows us to bound the total sampling cost and to readily verify the success of the procedure.

We are pleased to welcome Christian Schilling (LMU, Munich, Germany) and some of his group members (Julia Liebert and Lexin Ding) to the Laboratoire de Chimie Quantique de Strasbourg, on December 20th. On that occasion, we organise a mini-workshop on quantum information, electron correlation, and excited states. A detailed program is given below.

Venue: Salle Demuynck, Laboratoire de Chimie Quantique, Institut Le Bel, second floor.

Date: Wednesday,December 20th, 2023.

Contact: Emmanuel Fromager[fromagereVEDunistra.fr] and Saad Yalouz [syalouzVEDunistra.fr] (replace VED by @).

Morning session

——————————————————————————————————————————— 

09:30-10:30 Christian Schilling (Arnold Sommerfeld Centre for Theoretical Physics, LMU Munich)

The Electron Correlation Problem from a Quantum Information Perspective 

——————————————————————————————————————————— 

10:30-11:00 Coffee break

——————————————————————————————————————————— 

11:00-12:00 Lexin Ding (Arnold Sommerfeld Centre for Theoretical Physics, LMU Munich)

Quantum Information-Assisted Complete Active Space Optimization

Afternoon session

——————————————————————————————————————————— 

14:00-15:00 Julia Liebert (Arnold Sommerfeld Centre for Theoretical Physics, LMU Munich)

Functional theory for exited states: Foundations, Hartree-Fock theory and incorporation of the electron spin

The LCQS welcomed Paul Johnson as visiting Professor during the second week of September 2023.

---

Paul A. Johnson is an Associate Professor at the University of Laval in Québec. His research deals with strong electron correlation and seniority-zero wave functions. You can found more informations about him on his webpage.

He will give a seminar Tuesday, September 12, 2023 on ‘Strong Electronic Correlation with Richardson-Gaudin States‘

Abstract :

Weakly-correlated electronic systems are well-described as individual electrons: the wavefunction is a Slater determinant of the occupied orbitals with small corrections from single- and double-excitations. This is not the case for strongly-correlated systems. The wavefunction is very complicated in terms of Slater determinants and thus the correct physical picture is not independent electrons.

For strongly-correlated molecular systems, we have shown that the eigenvectors of reduced Bardeen-Cooper-Schrieffer Hamiltonians, so-called Richardson-Gaudin (RG) states, are a much better starting point. They describe weakly-correlated \emph{pairs} of electrons. They are tractable variationally and form a basis for the Hilbert space allowing for systematic improvement.

We will show for the isomers of H$_{10}$ that a single RG state is a very good approximation to the wavefunction. Corrections for the remaining weak electronic correlation are obtainable with an approximate functional, or with a short CI expansion in RG states. Thus, the present development is an analogue of both Kohn-Sham DFT and correlated wavefunction theories based on Slater determinant references. 

We are pleased to welcome Carmen Calzado and Celestino Angelias jury members of Pablo’s thesis for a mini-symposium, Thursday September 7th in the morning.

• Carmen Calzado is a Full Professor at University of Sevilla in Spain. She will talk about “Spin-crossover complexes on surface: a theoretical approach”.
• Celestino Angeli is an Associate Professor at University of Ferrara in Italy. He will talk about “Entanglement in Quantum Chemistry: the role of the molecular orbitals”.