615 research outputs found
Optimizing Jastrow factors for the transcorrelated method
We investigate the optimization of flexible tailored real-space Jastrow factors for use in the transcorrelated (TC) method in combination with highly accurate quantum chemistry methods, such as initiator full configuration interaction quantum Monte Carlo (FCIQMC). Jastrow factors obtained by minimizing the variance of the TC reference energy are found to yield better, more consistent results than those obtained by minimizing the variational energy. We compute all-electron atomization energies for the challenging first-row molecules C2, CN, N2, and O2 and find that the TC method yields chemically accurate results using only the cc-pVTZ basis set, roughly matching the accuracy of non-TC calculations with the much larger cc-pV5Z basis set. We also investigate an approximation in which pure three-body excitations are neglected from the TC-FCIQMC dynamics, saving storage and computational costs, and show that it affects relative energies negligibly. Our results demonstrate that the combination of tailored real-space Jastrow factors with the multi-configurational TC-FCIQMC method provides a route to obtaining chemical accuracy using modest basis sets, obviating the need for basis-set extrapolation and composite techniques
Exoteric effects at nanoscopic interfaces - Uncommon negative compressibility of nanoporous materials and unexpected cavitation at liquid/liquid interfaces
This PhD thesis is devoted to the investigation of some peculiar effects happening at nanoscopic interfaces between immiscible liquids or liquids and solids via molecular dynamics simulations. The study of the properties of interfaces at a nanoscopic scale is driven by the promise of many interesting technological applications, including: a novel technology for developing both eco-friendly energy storage devices in the form of mechanical batteries, as well as energy dissipation systems and, in particular, shock absorbers for the automotive market; biomedical applications related to cavitation, such as High-Intensity Focused Ultrasound (HIFU) ablation of cancer tissues and localised drug delivery, and many more. The kinetics of phenomena taking places at these scales is typically determined by large free-energy barriers separating the initial and final states, and even intermediate metastable states, when they are present. Because of such barriers, the phenomena we are interested in are "rare events", i.e. the system attempts the crossing of the barrier(s) many times before finally succeeding when an energy fluctuation makes it possible. At the same time, the magnitude of the barrier is determined by the energetics and dynamics of atoms, which forces us to model the system by taking into account both the femtosecond atomistic timescale and the timescale of the relevant phenomena, typically exceeding the former by several orders of magnitude. These longer timescales are inaccessible to standard molecular dynamics, so, in order to tackle this issue, advanced MD techniques need to be employed.
The thesis is divided into two parts, corresponding to the main lines of research investigated, which are (I) the interfaces between water and complex nanoporous solids, and (II) planar solid-liquid and liquid-liquid interfaces. Anticipating some results, atomistic simulations helped uncovering the microscopic mechanism behind the (incredibly rare!) giant negative compressibility exhibited by the ZIF-8 metal organic framework (MOF) upon water intrusion. Molecular dynamics simulations also supported experimental results showing how it is possible to change the intermediate intrusion-extrusion performance of ZIF-8 by changing its grain morphology and arrangement, from a fine powder to compact monolith. Free-energy MD calculations allowed to explain the exceptional stability of surface nanobubbles in water, at undersaturated conditions, on a surprisingly wide variety of substrates, characterized by disparate hydrophobicities and gas affinities; and yet, how they catastrophically destabilize in organic solvents. Finally, through simulations, some light was shed upon the working mechanism behind the novelly discovered phenomenon of how the interface between two immiscible liquids can act as a nucleation site for cavitation
University of Windsor Graduate Calendar 2023 Spring
https://scholar.uwindsor.ca/universitywindsorgraduatecalendars/1027/thumbnail.jp
Electron Thermal Runaway in Atmospheric Electrified Gases: a microscopic approach
Thesis elaborated from 2018 to 2023 at the Instituto de AstrofĂsica de AndalucĂa under the supervision of Alejandro Luque (Granada, Spain) and Nikolai Lehtinen (Bergen, Norway). This thesis presents a new database of atmospheric electron-molecule collision cross sections which was published separately under the DOI :
With this new database and a new super-electron management algorithm which significantly enhances high-energy electron statistics at previously unresolved ratios, the thesis explores general facets of the electron thermal runaway process relevant to atmospheric discharges under various conditions of the temperature and gas composition as can be encountered in the wake and formation of discharge channels
University of Windsor Graduate Calendar 2023 Winter
https://scholar.uwindsor.ca/universitywindsorgraduatecalendars/1026/thumbnail.jp
ASTRA: a Transition-Density-Matrix Approach to Molecular Ionization
We describe ASTRA (AttoSecond TRAnsitions), a new close-coupling approach to
molecular ionization that uses many-body transition density matrices between
ionic states with arbitrary spin and symmetry, in combination with hybrid
integrals between Gaussian and numerical orbitals, to efficiently evaluate
photoionization observables. Within the TDM approach, the evaluation of
inter-channel coupling is exact and does not depend on the size of the
configuration-interaction space of the ions. Thanks to these two crucial
features, ASTRA opens the way to studying highly correlated and comparatively
large targets at a manageable computational cost. Here, ASTRA is used to
predict the parameters of bound and autoionizing states of the boron atom and
of the N molecule, as well as the total photoionization cross section of
boron, N and formaldehyde, HCO. Our results are in excellent agreement
with available theoretical and experimental values from the literature. As a
proof of principle of ASTRA's ability to tackle larger targets, we report
preliminary results for the photoionization cross section of
magnesium-porphyrin (MgHCN), a biologically relevant
metallorganic complex with as many as 37 atoms.Comment: 30 pages, 8 figure
Development of numerical tools for the simulation, design and optimization of a Helicon Plasma Thruster
This work thesis focuses on the Helicon Plasma Thruster (HPT) as a candidate for generating thrust for small satellites and CubeSats. Two main topics are addressed: the development of a Global Model (GM) and a 3D self-consistent numerical tool. The GM is suitable for preliminary analysis of HPTs with noble gases such as argon, neon, krypton, and xenon, and alternative propellants such as air and iodine. A lumping methodology is developed to reduce the computational cost when modelling the excited species in the plasma chemistry. A 3D self-consistent numerical tool is also developed that can treat discharges with a generic 3D geometry and model the actual plasma-antenna coupling. The tool consists of two main modules, an EM module and a FLUID module, which run iteratively until a steady state solution is converged. A third module is available for solving the plume with a simplified semi-analytical approach, a PIC code, or directly by integration of the fluid equations. Results obtained from both the numerical tools are benchmarked against experimental measures of HPTs or Helicon reactors, obtaining very good qualitative agreement with the experimental trend for what concerns the GM, and an excellent agreement of the physical trends predicted against the measured data for the 3D numerical strategy
Investigating the Dynamics and Fragmentation of Nitroaromatic Radical Cations Through Femtosecond Time-Resolved Mass Spectrometry and Computational Chemistry
Chemists have sought to control molecular dissociation with lasers for decades. Effective control of unimolecular dissociation was only achieved with the development of high-intensity ultrashort pulsed lasers and coherent control techniques that operate on timescales faster than vibrational energy redistribution. In view of this, our lab has specialized in the study of polyatomic radical cations using femtosecond time-resolved mass spectrometry (FTRMS). The interest in radical cations stems from the fact that they are highly reactive species that contribute to many physical, chemical, and biological processes. For instance, radical cations participate in shock initiation of detonated energetic materials used as explosives and propellants. In this regard, we have studied some nitroaromatic radical cations commonly used as models for energetic materials. We discuss some results involving the dynamics of vibrational wave packets and rearrangement/fragmentation pathways.
Concerning vibrational wave packet dynamics, we employed computational chemistry to predict the most efficient probe wavelength for our experimental measurements on nitrobenzene cation demonstrating the feasibility and convenience of this approach. We also investigated pump-probe control schemes to manipulate fragmentation product yields in p-nitrotoluene (PNT) cation.
Finally, we investigated the dissociation dynamics and fragmentation pathways of o-nitroaniline, a model compound for the military explosive 2,4,6-triamino-1,3,5-trinitrobenzene (TATB). This model seems to capture the hydrogen bonding features that lead to energetically unfavorable rearrangement/fragmentation pathways in TATB. We expect that our experimental and computational results provide insights into the inherent stability of this molecule that explains the low sensitivity (and therefore relatively high safety) of TATB as an explosive
Electron Momentum Distributions from Strong-Field-Induced Ionization of Atoms and Molecules
High-intensity femtosecond laser pulses in the visible or infrared range can induce electron emission. This single-ionization process may be interpreted as a sequence of (nonadiabatic) tunnel ionization and subsequent acceleration of the electron by the external oscillating field in the presence of the electrostatic force between electron and parent ion. Based on the analysis of photoelectron momentum distributions from the numerical solution of the time-dependent Schrödinger equation, this thesis theoretically studies a variety of phenomena taking place in atoms as well as in molecules in strong fields. The underlying physical mechanisms are revealed by simplified models which take the nonperturbative character of the ionization process into account.
The simulation results for several settings are directly compared to measurements, offering the possibility to benchmark state-of-the-art theory and experiment against each other. One example of this is an investigation of the nonadiabatic strong-field ionization of atomic hydrogen in an attoclock setting. More generally, the deflection of the photoelectrons is analyzed in different attoclock configurations to explore the initial conditions of electrons at the tunnel exit—the position where the electron appears after tunneling. When a molecule is ionized, its orbital structure influences the liberated electron wave packet. The orbital imprint on the momentum-space phase of the wave packet, which encodes spatial information, is demonstrated and an interferometric approach to access these phases is evaluated. A characterization of the freed wave packet is crucial as it influences subsequent processes.
Such secondary processes are induced when the electron is driven back to the parent ion and scatters off. Similar to focusing of light by a lens, the Coulomb attraction forces scattered electron wave packets through focal points, causing a shift of their phase. Due to the interference of outgoing waves, these phases become visible in electron momentum distributions. For a faithful description, these focal-point effects must be included in a prefactor of the exponentiated action in semiclassical models. Furthermore, the control of electron scattering dynamics is demonstrated for low-energy electrons close to the continuum threshold by means of near-single-cycle terahertz pulses. The temporally-localized preparation of the electron wave packet by a femtosecond laser pulse at a well-defined time within the terahertz field enables a switching between different regimes of dynamics, ranging from recollision-free acceleration to extensive scattering phenomena.
In contrast to most studies in the electric dipole approximation that consider only the temporal evolution of the external electric field, various beyond-dipole effects in strong-field ionization are explored in the present work. The microscopic mechanisms of nondipole modifications are thoroughly analyzed. There, the effects of the spatially-varying electric field and of the magnetic field as well as their fingerprints on the geometry of the momentum distributions are identified. Furthermore, the subcycle time resolution of the light-induced momentum transfer in an attoclock-like setup is explored theoretically. Electron recollisions entirely change the observed nondipole effects and render the observations sensitive to the electronic target structure. The high-order above-threshold ionization caused by large-angle scattering is investigated both for exemplary atoms and for diatomic molecules through examination of nondipole shifts of the lateral momentum distribution. The phases of the electron wave packets are also altered by beyond-dipole effects. It is shown that this results in a displacement of ring-link structures known as above-threshold ionization rings that are caused by intercycle interference. In addition, the holographic structures arising from the subcycle interference of scattered and nonscattered electrons are modified
- …