Correlated electronic structure theory for challenging systems

The photochemistry of molecules can be investigated computationally, and this provides great insight into the underlying chemistry and physics. Such computational approaches are challenging and can pose many difficulties compared to ground state methodologies. Care must be taken to accurately describe these systems, as some lowlevel approximate methods can fail. The geometrical and electronic structures (TiO2)n clusters (n=1-4) have been investigated. These are of enormous technological interest as wide band-gap semiconductors yet the nature of electronic transitions in nano-sized clusters has yet to be fully elucidated. Structures of the neutral closed-shell, radical cationic and radical anionic clusters at each size are described and rationalised in terms of the pseudo-Jahn- Teller effect. We have used high-level response theory to set benchmarks for such systems. The TiO2 monomer is the simplest of the clusters studied yet proves a stern test for many lower order ab-initio methods. It is shown that high-level methods are required to properly describe this simple molecule. The Monte Carlo Configuration Interaction method attempts to combine the power of Full CI with a scalability that allows it to be used to study much larger systems. It can be systematically improved and can approach the accuracy of the Full CI method. This method is applied here to investigate potential energy surfaces and multipole moments of a range of small but challenging systems

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

Some experimental and theoretical aspects of structure and bonding as studied by ESCA

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Development of highly efficient and accurate real-space integration methods for Hartree-Fock and hybrid density functional calculations

The central focus of molecular electronic structure theory is to find approximate solutions to the electronic Schrödinger equation for molecules, and as such represents an essential part of any theoretical (in silico) study of chemical processes. However, a steep increase of the computational cost with increasing system size often prevents the application of accurate approximations to the molecules of interest. The main focus of the present work is the efficient evaluation of Fock-exchange contributions, which typically represents the computational bottleneck in Hartree-Fock (HF) and hybrid density functional theory (DFT) calculations. This bottleneck is addressed by means of seminumerical integration, i.e., one electronic coordinate within the 4-center-2-electron integral tensor is represented analytically and one numerically. In this way, an asymptotically linear scaling method for computing the exchange matrix (denoted as sn-LinK) is developed, enabling fast and accurate ab-initio calculations on large molecules, comprising hundreds or even thousands of atoms, even in combination with large atomic orbital basis sets. The novel sn-LinK method comprises improvements to the numerical integration grids, a rigorous, batch-wise integral screening scheme, the optimal utilization of modern, highly parallel compute architectures (e.g., graphics processing units; GPUs), and an efficient combination of single- and double-precision arithmetic. In total, these optimizations enable over two orders of magnitude faster evaluation of Fock-exchange contributions. Consequently, this greatly improved performance allows to perform previously unfeasible computations, which is also demonstrated at the example of an ab initio molecular dynamics simulation (AIMD) study on the hydrogen bond strengths within double-stranded DNA. In addition to Fock-exchange, the other two computational bottlenecks in hybrid-DFT applications – the evaluation of the Coulomb potential and the numerical integration of the semilocal exchange-correlation functional – are also addressed. Finally, more efficient methods to evaluate more accurate post-HF/DFT methods, namely the random-phase approximation (RPA) and the second-order approximate coupled cluster (CC2) method, are also put forward. In this way, the highly efficient methods introduced in this thesis cover some of the most substantial computational bottlenecks in electronic-structure theory – the evaluation of the Coulomb- and the exchange-interactions, the integration of the semilocal exchange-correlation functional, and the computation of post-Hartree-Fock correlation energies. Consequently, computational chemistry studies on large molecules (>100 atoms) are accelerated by multiple orders of magnitude, allowing for much more accurate and thorough in-silico studies than ever before

Time-resolved photoelectron imaging of model biological chromophores

Investigating the underlying forces that have a key role in light-matter interactions is crucial to understanding many fundamental processes that occur in nature. This thesis describes a series of experiments investigating model biological chromophores, providing an insight into the photochemistry of “ultraviolet selfprotection” schemes, invoked in many molecules involved in the protection of flora and fauna from the potentially damaging effects of excess ultraviolet radiation. These experiments are achieved through the application of time-resolved photoelectron imaging (TR-PEI) supported by high-level computational chemistry calculations. This thesis will set out both the motivation for the work, consisting of a comprehensive literature review of the subject, as well as an in-depth description of the optical set-up, laser system and spectrometer, as well as non-trivial data handling approaches involved in performing the experiments. This will lead on to work concerning the model chromophore aniline (and several aniline derived systems), guaiacol and finally a series of tertiary aliphatic amines. To conclude, various potential avenues of future work are discussed, considering both the experimental methodology and possible molecules of interest building upon the work described within this thesis

LASER SPECTROSCOPY OF RADICALS CONTAINING GROUP IIIA AND VA ELEMENTS

Radicals are interesting to study because of importance in so many processes such as semiconductor growth or stellar evolution. Laser induced fluorescence (LIF) and wavelength resolved emission spectra of jet cooled HPS, HAsO, AsD2, H2PS, and F2BO have been measured using the pulsed discharge jet technique. Several bands in the à 1A′′ − X̃ 1A′ transition of HPS were observed and assigned with the help of ab initio calculations. The ab initio geometries showed that HPS does not follow Walsh’s predictions for the angle change upon electronic excitation; Walsh predicts an increase in HPS upon excitation while a decrease is calculated. Ab initio Walsh-style orbital angular correlation diagrams for both electronic states show a change in correlation for some orbitals upon electronic excitation, an effect that Walsh did not predict. The à 1A′′ − X̃ 1A′ transitions were measured in HAsO and DAsO for the first time. A molecular geometry was derived for each electronic state from experimental rotational constants. The experimental geometries prove that HAsO also violates Walsh’s rules for the same reason shown in HPS. The à 2A1 – X̃ 2B1 electronic transition of AsD2 and AsHD were measured. Vibrational levels observed in emission were fit to a local mode vibrational Hamiltonian. Using the previously reported rotational constants for AsH2 and those determined for AsD2 in this work, an improved estimate of the excited state geometry was obtained. The discovery of the B̃ 2A′ − X̃ 2A′ band system of H2PS is the first report of this molecule. Both D2PS and HDPS were also observed. Ab initio calculations helped assign the transition. H2PS is one of the few tetra-atomic or larger molecules that violates Kasha’s empirical rule due to the large separation between the B̃ and à states. Finally, laser induced fluorescence spectra of the F2BO radical was observed for the first time. Previous work showed two band systems with only a tentative assignment. The measured LIF spectra confirm the identity of the two band systems as the B̃ 2A1 – X̃ 2B2 and the B̃ 2A1 – à 2B1 transitions showing F2BO also violates Kasha’s rule

Multiscale Study of BaTiO3 Nanostructures and Nanocomposites

Advancements in integrated nanoelectronics will continue to require the use of unique materials or systems of materials with diverse functionalities in increasingly confined spaces. Hence, research on finite-dimensional systems strive to unearth and expand the knowledge of fundamental physical properties in certain key materials which exhibit numerous concurrent and exploitable functions. Correspondingly, ferroelectric nanostructures, which particularly display a plethora of complex phenomena, prevalent in countless fields of research, are noteworthy candidates. Presently, however, the assimilation of zero-(0D) and one-dimensional (1D) ferroelectric into micro- or nano-electronics has been lagging, in part due to a lack of applied and fundamental studies but also due to the paucity of synthetic strategies yielding high quality monocrystalline structures. In this work, the problematics of size reduction, which affects many aspects of electronic devices, was addressed. Furthermore, the depolarizing effects associated with finite thickness in ferroelectric nanostructures was investigated in connection with other crucial boundary conditions. The work reported in this dissertation concerned isolated 0D and 1D BaTiO3 nanocrystals and nanocomposites composed of periodic arrays of BaTiO3 nanowires embedded in a matrix formed by another ferroelectric material. A systematic investigation was conducted for those three types of nanostructures from a quantum mechanical and atomistic perspective using both direct-first-principles and first-principles-derived methods. Using first-principles-based calculations, the structural phase sequences in 0D (cubic-to-tetragonal-to-monoclinic-to-rhombohedral) and 1D (cubic-to-tetragonal-to-orthorhombic-to-monoclinic) BaTiO3 nanoparticles revealed differences from that of the bulk and thin film systems. The monoclinic symmetry found in the 0D compounds, and as for the ground-state of 1D systems, were also affected by size effects and tuned by varying parameters related to the depolarizing effect. Strong electromechanical responses characteristic to the monoclinic symmetry, were also found. In addition, by partially screening the uncompensated charges at the surface of the nanodots, a small range existed (∼87% to ∼95% screening) where both the polarization and toroidal moment coexisted within the nanoparticles. Ferroelectric $nanocomposites$ are novel systems that were also examined and were found to exhibit completely original properties not yet observed in either constituents alone. The temperature-dependent properties such as the structural phases and behavior of the polarization within these nanocomposites were obtained. Interesting new features related to flux-closure configurations were discovered. Transitions associated with the cores of electric dipole vortices were correlated to the direction of in-plane polarization. In addition, vortex-antivortex pairs in a peculiar phase-locked configuration were ascertained in these structures. Complementary density-functional theory calculations were also performed for BaTiO3 nanowires with dissociated-water adsorbates as a function of the out-of-plane lattice constant. Topological defects with winding numbers ranging from 1 to -3 were found in the water-covered nanowires. The ground-state was found to be of triclinic symmetry. Ab-initio calculations were also performed for nanocomposites to investigate the electronic properties of the phase-locked configuration. Similarly to the Monte-Carlo simulations, a configuration containing both vortices (not localized in the nanowires though) and antivortices was found to be the ground state. Mastery of nanomaterials requires merging theoretical research with experimental observation, hence a synthesis project was developed to obtain BaTiO3 nano-tubes and wires using direct pore filling of nanoporous templates. The preliminary results suggested the synthesis of polycrystalline nanostructures depend on the template pore surface polarity and size. The results presented in this dissertation suggested that ferroelectric nanostructures continue to be of great fundamental value and may substantially impact advancement in certain technologies. Furthermore, the work on nanocomposites offered a glimpse to the novel functionalities in ferroelectrics

Anion Photoelectron Spectroscopic Studies: Antioxidants, Actinide Clusters, and Molecular Activation

Gas phase anion photoelectron spectroscopy is uniquely suited to study chemistry at the molecular level, as atoms, molecules, and clusters are isolated and thus unperturbed by confounding environmental effects which often complicate analyses carried out in the liquid or solid state. Photoelectron spectroscopy provides information about the electronic structure of anions, as well as the geometry of the anions and corresponding neutral species when combined with theoretical calculations. A variety of ion sources were employed to generate the anions in these studies: electrospray ionization (ESI), laser vaporization (LVS), and pulsed arc cluster ion source (PACIS). Using these techniques, two antioxidants, a range of actinide containing clusters, and multiple activation reactions were studied. Additionally, a new double rod laser vaporization source was designed and constructed to generate single atom catalyst (SAC) mimics. Chapter III presents the studies of this thesis and is divided into three major sections based on ion source: ESI, LVS, and PACIS. ESI brought the water-soluble antioxidants (ascorbate, deprotonated ascorbate, propyl gallate, and gallate) into the gas phase. LVS ablated uranium and thorium rods to generate gas phase atoms and actinide containing clusters, as well as highlighted the reaction between iridium and hydroxylamine and the phenomenon of intramolecular electron-induced proton transfer. Finally, using PACIS, two thorium clusters were generated, and CO2 activation with two metal hydrides were studied