On the mass of atoms in molecules: Beyond the Born-Oppenheimer approximation

Describing the dynamics of nuclei in molecules requires a potential energy surface, which is traditionally provided by the Born-Oppenheimer or adiabatic approximation. However, we also need to assign masses to the nuclei. There, the Born-Oppenheimer picture does not account for the inertia of the electrons and only bare nuclear masses are considered. Nowadays, experimental accuracy challenges the theoretical predictions of rotational and vibrational spectra and requires to include the participation of electrons in the internal motion of the molecule. More than 80 years after the original work of Born and Oppenheimer, this issue still is not solved in general. Here, we present a theoretical and numerical framework to address this problem in a general and rigorous way. Starting from the exact factorization of the electron-nuclear wave function, we include electronic effects beyond the Born-Oppenheimer regime in a perturbative way via position-dependent corrections to the bare nuclear masses. This maintains an adiabatic-like point of view: the nuclear degrees of freedom feel the presence of the electrons via a single potential energy surface, whereas the inertia of electrons is accounted for and the total mass of the system is recovered. This constitutes a general framework for describing the mass acquired by slow degrees of freedom due to the inertia of light, bounded particles. We illustrate it with a model of proton transfer, where the light particle is the proton, and with corrections to the vibrational spectra of molecules. Inclusion of the light particle inertia allows to gain orders of magnitude in accuracy

Electronic density response to molecular geometric changes from explicit electronic susceptibility calculations

We present a first principles approach to compute the response of the molecular electronic charge distribution to a geometric distortion. The scheme is based on an explicit representation of the linear electronic susceptibility. The linear electronic susceptibility is a tensor quantity which directly links the first-order electronic response density to the perturbation potential, without requiring self-consistency. We first show that the electronic susceptibility is almost invariant to small changes in the molecular geometry. We then compute the dipole moments from the response density induced by the geometrical changes. We verify the accuracy by comparing the results to the corresponding values obtained from the self- consistent calculations of the ground-state densities in both geometries

Effect of puckering motion and hydrogen bond formation on the vibrational circular dichroism spectrum of a flexible molecule: the case of ( S )-1-indanol

International audienceThe influence of flexibility and hydrogen bond formation on the IR absorption and vibrational circular dichroism (VCD) spectrum of a floppy protic molecule, namely, (S)-1-indanol, is studied in both non-polar CCl 4 and polar DMSO solvents. The experimental IR absorption and VCD spectra obtained by Fourier transform spectroscopy are interpreted using both static density functional theory (DFT) calculations and first principles molecular dynamics (FPMD) within DFT, using the nuclear velocity perturbation theory (NVPT). Simulation of the spectra based on static optimised geometries is not sufficient in CCl 4 and going beyond static calculations is mandatory for satisfactorily reproducing the VCD spectra. The FPMD results obtained in DMSO indicate that (S)-1-indanol is hydrogen-bonded to one DMSO molecule. As a result, static "cluster-in-the-bulk" DFT calculations in which the solute-solvent interaction is modeled as the most stable (S)-1-indanol:DMSO-d6 complexes in a DMSO continuum yield satisfactory agreement with the experiment. Correspondence between experimental and simulated spectra is slightly improved when the VCD spectrum is calculated as the summed contributions of snapshots extracted from FPMD trajectories, due to better sampling of the potential-energy surface. Finally, NVPT calculations further improve the description of experimental spectra by taking into account higher-energy structures, which are not necessary local minima

The WMT'18 Morpheval test suites for English-Czech, English-German, English-Finnish and Turkish-English

Peer reviewe

Democratizing Neural Machine Translation with OPUS-MT

This paper presents the OPUS ecosystem with a focus on the development of open machine translation models and tools, and their integration into end-user applications, development platforms and professional workflows. We discuss our on-going mission of increasing language coverage and translation quality, and also describe on-going work on the development of modular translation models and speed-optimized compact solutions for real-time translation on regular desktops and small devices

Théorie des perturbations de la fonctionnelle de densité pour la modélisation des interactions faibles et de la spectroscopie en phase condensée

This thesis deals with the development and application of computational methods for the efficient and accurate calculation of spectroscopic parameters and non-covalent inter-molecular interactions in condensed-phase systems from quantum chemical methods. Specifically, electronic current densities and polarizability effects are computed using density functional perturbation theory. The nuclear velocity perturbation theory is rigorously derived from the exact factorization of the electron-nuclear wave function. Its implementation within a large-scale electronic structure program package is reported and the calculation of dynamical vibrational circular dichroism in the condensed phase is demonstrated. A position-dependent mass of nuclei in molecules is derived, addressing the fundamental questions as to how masses move in a molecule. First steps towards a density-based modeling of inter-molecular interactions using a compact representation of the electronic susceptibility are devised.Cette thèse porte sur l'étude des interactions faibles et de la spectroscopie vibrationnelle en phase condensée à partir d'un développement théorique basé sur la théorie de la perturbation de la fonctionnelle de densité. D'une part des corrections de la fonction d'onde Born-Oppenheimer ont été calculées pour déterminer le moment magnétique induit par les vibrations et ainsi calculer des spectres de dichroïsme circulaire vibrationnel. D'autre part, une modélisation des effets de polarisation est réalisée à l'aide d'une nouvelle représentation de la susceptibilité électronique non-locale

The University of Helsinki and Aalto University submissions to the WMT 2020 news and low-resource translation tasks

This paper describes the joint participation of University of Helsinki and Aalto University to two shared tasks of WMT 2020: the news translation between Inuktitut and English and the low-resource translation between German and Upper Sorbian. For both tasks, our efforts concentrate on efficient use of monolingual and related bilingual corpora with scheduled multi-task learning as well as an optimized subword segmentation with sampling. Our submission obtained the highest score for Upper Sorbian -> German and was ranked second for German -> Upper Sorbian according to BLEU scores. For English-Inuktitut, we reached ranks 8 and 10 out of 11 according to BLEU scores.Peer reviewe

Iterative approach for the moment representation of the density-density response function

The linear density-density response function χ(r,r′) can be transformed from its Eigensystem representation into a computationally more efficient moment expansion representation using a suitable unitary transformation. Here, we propose an iterative approach for the direct calculation of this moment representation without resorting to either the direct-space or the conventional Eigensystem representation

Proceedings of the Third Conference on Machine Translation (WMT) : Shared Task Papers

Progress in the quality of machine translation output calls for new automatic evaluation procedures and metrics. In this paper, we extend the Morpheval protocol introduced by Burlot and Yvon (2017) for the English-to-Czech and English-to-Latvian translation directions to three additional language pairs, and report its use to analyze the results of WMT2018’s participants for these language pairs. Considering additional, typologically varied source and target languages also enables us to draw some generalizations regarding this morphology-oriented evaluation procedure.</div