Seminars

Friday 20 Octobre, at 14h00
On Zoom.

Accelerating quantum-chemistry wave-function calculations using density-functional theory.

Julien Toulouse
Laboratoire de Chimie Théorique - Sorbonne Université & CNRS, 4 place Jussieu, Paris
Institut Universitaire de France

There are two main families of quantum-chemistry electronic-structure computational approaches: wave-function theory (WFT) and density-functional theory (DFT). WFT methods (such as configuration interaction and coupled cluster) directly attack the calculation of the correlated many-particle wave function. They can be accurate and systematically improvable but they suffer from a large computational cost, in particular due to a slow convergence with respect to the size of the one-particle basis set. DFT avoids the calculation of the correlated wave function by using a functional of the one-particle density. DFT is computationally efficient, in particular thanks to a fast basis-set convergence, but suffers from uncontrolled errors due to the use of approximate density functionals.

After reviewing these two main families of electronic-structure computational approaches, I will present a recently developed approach [1,2] for accelerating the basis-set convergence of WFT by using DFT for correcting for the incompleteness of the one-particle basis set, with the goal of obtaing a method which is accurate, systematically improvable, and yet computationally efficient. I will also show numerical results of this approach on atomic and molecular systems [3,4].

References

[1] E. Giner, B. Pradines, A. Ferté, R. Assaraf, A. Savin, J. Toulouse, Curing basis-set convergence of wave-function theory using density-functional theory: A systematically improvable approach, Journal of Chemical Physics 149, 194301 1-15 (2018).
[2] P.-F. Loos, B. Pradines, A. Scemama, J. Toulouse, E. Giner, A density-based basis-set correction for wave function theory, Journal of Physical Chemistry Letters 10, 2931-2937 (2019).
[3] E. Giner, A. Scemama, J. Toulouse, P.-F. Loos, Chemically accurate excitation energies with small basis sets, Journal of Chemical Physics 151, 144118 1-10 (2019).
[4] E. Giner, A. Scemama, P.-F. Loos, J. Toulouse, A basis-set error correction based on density-functional theory for strongly correlated molecular systems, Journal of Chemical Physics 152, 174104 1-16 (2020).

Friday 7 February, at 10h
Room A018, building 100, IJCLab.

Towards high-precision ab initio calculations over the nuclear chart.

Pierre Arthuis
University of Surrey, UK.

Over the past few decades, ab initio many-body methods for finite nuclei have moved past the limitations constraining them to the few-body sector, and are now reaching nuclei up to A~130 thanks to both a flourishing in new formalisms and computational progress. While progress keep being made in terms of accessible systems and observables, new constraints have appeared that prevent them to enter into the heavy mass regime.

In this seminar, I will discuss what prevents ab initio methods from moving past the Sn isotopic line, as well as recent attempts at extending this range, and explore approaches to circumvent those limitations and extend accurate ab initio calculations over the whole nuclear chart.