Published in Journal of Chemical Theory and Computation, 2018
We report unphysical irregularities and discontinuities in some key experimentally measurable quantities computed within the GW approximation of many-body perturbation theory applied to molecular systems.
Published in Journal of Chemical Theory and Computation, 2019
Quantum Package is an open-source programming environment for quantum chemistry specially designed for wave function methods.
Recommended citation: "Quantum Package 2.0" Yann Garniron, Thomas Applencourt, Kevin Gasperich, Anouar Benali, Anthony Ferté, Julien Paquier, Barthélémy Pradines, Roland Assaraf, Peter Reinhardt, Julien Toulouse, Pierrette Barbaresco, Nicolas Renon, Grégoire David, Jean-Paul Malrieu, Mickaël Véril, Michel Caffarel, Pierre-François Loos, Emmanuel Giner, Anthony Scemama Journal of Chemical Theory and Computation DOI:10.1021/acs.jctc.9b00176 http://mveril.github.io/files/2019-05-13-Quantum-Package-2_0.pdf
Published in WIREs Computational Molecular Science, 2021
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 (π → π, n → π, double excitation, Rydberg, singlet, doublet, triplet, etc.) in small- and medium-sized molecules.
Published in The Journal of Chemical Physics, 2021
Following our recent work on the benzene molecule [P.-F. Loos, Y. Damour, and A. Scemama, J. Chem. Phys. 153, 176101 (2020)], motivated by the blind challenge of Eriksen et al. [J. Phys. Chem. Lett. 11, 8922 (2020)] on the same system, we report accurate full configuration interaction (FCI) frozen-core correlation energy estimates for 12 five- and six-membered ring molecules
The accurate modelling of excited-state energies and properties with ab initio quantum chemistry methods is a clear ambition of the electronic structure theory community. One particularly interesting and challenging aspect of molecular excited states is their variety and the large panel of methods that have been created to compute their vertical transitions energies. To ease comparisons between this zoo of theoretical models, in the last few years, we have created a large set of highly accurate vertical excitation energies (currently more than 500) of various natures (π → π, n → π, double excitation, Rydberg, singlet, doublet, triplet, etc.) in small- and medium-sized molecules. These reference values have been obtained using an incremental strategy which consists in combining high-order coupled cluster and selected configuration interaction calculations using increasingly large basis sets in order to reach high accuracy. Thank to this systematic strategy, we have been able to produce theoretical best estimates (TBEs) with the aug-cc-pVTZ basis set for each of these transitions, as well as basis set corrected TBEs for some of them. The TBEs/aug-cc-pVTZ have been employed to benchmark a large number of (lower-order) wave function methods. 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. A live demonstration will be given during the present talk.
This poster session at the RCTF2018 was about my MSc work concerning the discontinuities in GW methods. This work is linked to the eponymous publication.
The presentation I made for the GMO group talked about the web demo I realised by using shellinabox, Docker and Ubuntu. Thanks to the demo you can try Quantum Package directly on your web browser before installing it.
This thesis concerns two main topics: i) molecular electronic excited states and ii) digital tools for theoretical quantum chemistry. The present thesis starts with a study of the GW family of approximations. These are known to be accurate and relatively cheap methodologies for the computation of charged excitations such as ionization potentials and electronic affinities in molecules and band gaps in solids. These methods can be perturbative, partially or fully self-consistent and are able to provide chemically accurate properties for weakly correlated systems. However, as discussed in details, discontinuities can be observed for several key properties, particularly in potential energy surfaces even in the weakly correlated regime. These discontinuities are due to poles in the self-energy — a key quantity of the GW formalism — as each of its branch is associated with a distinct solution of the quasiparticle equation. We show that, in diatomic molecules, multisolution behavior in frontier orbitals is more likely if the energy gap between these orbitals is small. The central part of this thesis deals with the QUEST project, a large database of more than 600 vertical excitations energies of various natures. The aim of this open-source and easily-modifiable database is to provide highly-accurate reference excitations energies for benchmarking and cross comparisons of computational models. These reference excitations energies are divided into six subsets depending on the size of the molecules and the types of excitations. In order to gather the huge amount of data of the QUEST project, we have created a 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. The next part is about digital tools and, in particular, the implementation of a web demo of Quantum Package, an open-source software developed primarily by our research group, which has been designed to be easy to use by quantum chemists, whether they are users or developers. The possibility of testing the software without the need to install it is a real opportunity to make Quantum Package more popular in our community. The containerization technology provided by Docker and the Linux kernel allow us to quickly start a clean demo environment for each user and stop it easily with interesting performance. Finally, pursuing the recent work of our team on the benzene molecule, we report reference frozen-core correlation energies for the twelve five- and six-membered ring molecules of the QUEST project using the Configuration Interaction using a Perturbative Selection made Iteratively (CIPSI) algorithm in the standard correlation-consistent double-ζ Dunning basis set (cc-pVDZ). These correspond to Hilbert spaces with sizes ranging from 10²⁸ to 10³⁶. The performance and convergence properties of several series of methods are investigated. In particular, we study the convergence properties of the Møller-Plesset perturbation series up to fifth-order as well as various single-reference coupled-cluster models which include up to quadruple excitations.
