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Concept

Base (chimie quantique)

Une base en chimie quantique est un ensemble de fonctions utilisées afin de modéliser des orbitales moléculaires, qui sont développées comme combinaisons linéaires de telles fonctions avec des poids ou coefficients à déterminer. Ces fonctions sont habituellement des orbitales atomiques, car centrées sur les atomes, mais des fonctions centrées sur les liaisons ou les fonctions centrées des doublets non liants ont été utilisées comme l'ont été des paires de fonctions centrées sur les deux lobes d'une orbitale p. De plus, des bases composées d'ondes planes jusqu'à une longueur d'onde de coupure sont également utilisées, particulièrement dans les calculs sur des systèmes avec des conditions périodiques aux limites. Introduction En chimie numérique récente, les calculs quantiques sont typiquement effectués au moyen d'un ensemble fini de fonctions de base. Dans ces cas, les fonctions d'onde considérées sont toutes représentées comme des vecteurs, dont les composantes corresponde

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Publications associées (52)

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CH-353: Introduction to electronic structure methods

Repetition of the basic concepts of quantum mechanics and main numerical algorithms used for practical implementions. Basic principles of electronic structure methods:Hartree-Fock, many body perturbation theory, configuration interaction, coupled-cluster theory, density functional theory.

CH-431: Physical and computational organic chemistry

This course introduces modern computational electronic structure methods and their broad applications to organic chemistry. It also discusses physical organic concepts to illustrate the stability and reactivity of organic molecules.

PHYS-724: Ultrafast Phenomena

The course will cover fundamental concepts and recent developments in the field of ultrafast spectroscopy and introduce the basic theory to understand ultrafast (10-16 - 10-9 s) phenomena in chemistry, biology and condensed matter physics.

Méthode de Hartree-Fock

En physique et chimie numérique, la méthode de Hartree-Fock est une méthode de résolution approchée de l'équation de Schrödinger d'un système quantique à plusieurs corps utilisant le principe variatio

Chimie numérique

La chimie numérique ou chimie informatique, parfois aussi chimie computationnelle, est une branche de la chimie et de la physico-chimie qui utilise les lois de la chimie théorique exploitées dans de

Orbitale moléculaire

vignette|Orbitales moléculaires du 1,3-butadiène, montrant les deux orbitales occupées à l'état fondamental : π est liante entre tous les atomes, tandis que π n'est liante qu'entre les atomes C et C a

Efficient Treatment of Correlation Energies at the Basis-Set Limit by Monte Carlo Summation of Continuum States

Martin Peter Bircher, Ursula Röthlisberger, Justin Villard

The calculation of electron correlation is vital for the description of atomistic phenomena in physics, chemistry, and biology. However, accurate wavefunction-based methods exhibit steep scaling and often sluggish convergence with respect to the basis set at hand. Because of their delocalization and ease of extrapolation to the basis-set limit, plane waves would be ideally suited for the calculation of basis-set limit correlation energies. However, the routine use of correlated wavefunction approaches in a plane-wave basis set is hampered by prohibitive scaling due to a large number of virtual continuum states and has not been feasible for all but the smallest systems, even if substantial computational resources are available and methods with comparably beneficial scaling, such as the Moller-Plesset perturbation theory to second order (MP2), are used. Here, we introduce a stochastic sampling of the MP2 integrand based on Monte Carlo summation over continuum orbitals, which allows for speedups of up to a factor of 1000. Given a fixed number of sampling points, the resulting algorithm is dominated by a flat scaling of similar to O(N-2). Absolute correlation energies are accurate to

AMER CHEMICAL SOC2020

Chargement

Photophysics and Photochemistry from First Principles

Matteo Guglielmi

Quantum chemical methods are important tools for the predictions of electronic structure and energetics of molecules providing the interpretation of spectroscopic experiments and reaction mechanisms. In this thesis, density functional theory (DFT) and wavefunction-based methods are used in order to evaluate their ability in predicting experimental data with a particular attention to photoprocesses occurring in both gas and condensed phase systems. The cases studied in this work are inspired by problems both from biology and material science. In the first study, the photoexcitation properties of the molecule 7-hydroxyquinoline·(NH3)3, a prototypical molecular wire, are investigated. A concerted mechanism according to which all protons are transferred simultaneously in a fast process (∼ 100 fs) that amounts to the net transport of one proton from the oxygen to the nitrogen of 7-hydroxyquinoline is observed. In addition, the proton transfer pathway involves all three ammonia molecules and not only two as previously proposed. The presence of potential energy surface crossings with a dark-state is responsible for the low experimentally observed quantum-yield. To better understand the complex photophysics of the amino acid tryptophan, which is widely used as a probe of protein structure, we performed DFT and TDDFT calculations of gas phase tryptophan solvated with a controlled number of water molecules. The addition of two water molecules sufficiently lengthens the excited state lifetime enabling vibrationally resolved spectra. Quantum chemical calculations at the RI-CC2/aug-cc-pVDZ level, together with TDDFT/pw based first-principles MD simulations of the excited state dynamics, clearly demonstrate how interactions with water destabilize the photodissociative states and increase the excited state lifetime. Due to the availability of high-resolution experimental data, this system is ideally suited as benchmark model for the evaluation of the performance of different quantum chemical methods. An extensive low-energy isomer search was carried out for [TrpH·(H2O)n=0,1,2]+ using a hierarchy of theoretical methods ranging from classical force fields to a variety of DFT methods (BLYP, B3LYP, M05, M05-2X, M06, M06-2X and M06-HF with different basis sets) up to the CBS-C level. For the low-energy structures, the harmonic vibrational frequencies are calculated and compared with high resolution experimental IR spectra. It turns out that in all three cases, CBS-C is able to predict the lowest energy isomers that are in agreement with the experimentally observed vibrational spectra. In most cases, M06 provides energetics in close agreement with the CBS-C results. On the other hand, M05-2X is the only functional that yields highly-reliable predictions of the vibrational spectra. A natural extension of the tryptophan model to condensed phase was performed by an investigation of the [Trp-Lys]+ motif studied in the protein Human Serum Albumin via QM/MM simulations. Like in the case of gas phase [TrpH]+, solvation effects lead to an increase of the σN-H orbital energies. As a result, the first photoaccessible state of sW K+ is primarily a photostable π orbital, while in dW K+, photoexcitation leads to a dissociative pathway. The last application of this work concerns the optical properties of a novel dye for the sensitization of titanium dioxyde nanoparticles in Grätzel solar cells. The presence of a new absorption band in the visible region opens the way for a rational design of new dyes with improved properties.

EPFL2010

Chargement

Wannier90 as a community code: new features and applications

Marco Gibertini, Hyungjun Lee, Daniel John Gilles Marchand, Antimo Marrazzo, Nicola Marzari, Lorenzo Paulatto, Giovanni Pizzi, Samuel Poncé, Junfeng Qiao

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

2020

Chargement