Theory And Application Development Of Electronic Structure Methods Involving Heavy Computation

The propargyl radical, the most stable isomer of C3H3, is very important in combustion reactions. However, theoretical calculations have never been able to find a strong absorption around 242 nm as seen in experiments. In this study, we calculated the electronic energy levels of the propargyl radical using highly accurate multireference methods, including multireference configuration interaction singles and doubles method with triples and quadruples treated perturbatively [denoted as MRCISD(TQ)], as well as second and third order generalized Van Vleck perturbation theories (GVVPT2 and GVVPT3). Calculations indicate that this absorption can be solely attributed to a Franck-Condon-allowed transition from the ground B1 state to the Rydberg-like first A1 excited state. Calculations also show that GVVPT2 with a relatively small active space fails to capture enough Rydberg character of this excited state, while it can be recovered by GVVPT3, MRCISD, and MRCISD(TQ). In order to speed up MRCISD(TQ) calculations, the triple and quadruple (TQ) perturbative corrections, the most time-consuming part of MRCISD(TQ) calculations, were parallelized using Message Passing Interface (MPI). The MRCISD(TQ) method is organized in the framework of macroconfigurations, which allows the use of incomplete reference spaces and provides an efficient means of screening large number of non-interacting configuration state functions (CSFs). The test calculations show that the parallel code achieved close to linear speed-up when the number of CSFs in each macroconfiguration is small. The speed-up suffers when large numbers of CSFs exist in only a few macroconfigurations. The computer algorithm for second-order generalized van Vleck multireference perturbation theory (GVVPT2) was similarly parallelized using the MPI protocol, organized in the framework of macroconfigurations. The maximum number of CSFs per macroconfiguration is found to have less influence on the MPI speedup and scaling than in the case of MRCISD(TQ). It was previously found that unrestricted local density approximation (LDA) orbitals can be used in place of MCSCF to provide orbitals for GVVPT2. This inspired us to use the more controllable restricted density functional theory (DFT) to provide unbiased orbitals for GVVPT2 calculations. In this study, the relationship between restricted DFT and unrestricted DFT were explored and the restricted DFT results were obtained by utilizing subroutines from unrestricted DFT calculations. We also found that the DIIS technique drastically sped up the convergence of RDFT calculations. Plane wave DFT methods are commonly used to efficiently evaluate solid state materials. In this work, the electronic properties of pristine graphene and Zn-phthalocyanine tetrasulfonic acid (Zn-PcS) physisorbed on single-layer graphene were calculated using plane wave DFT. The Perdew-Burke-Ernzerhof functional with dispersion correction (PBE-D2) was used. The densities of states were obtained for both pristine and absorbed graphene, and the disappearance of the characteristic dip in the density of states of the adsorbed system was attributed to the lowest unoccupied molecular orbital of the adsorbed molecule. A small charge transfer from graphene to the molecule was found. We present comparison of DFT results with Scanning Tunneling Microscopy/Spectroscopy data

New Methods for Core-Hole Spectroscopy Based on Coupled Cluster Theory

X-ray absorption spectra (XAS) is a method used to investigate atomic local structure and electronic states. Coupled cluster method is a numerical method used for describing many-body systems and electron correlation in a wavefunction. When equation-of-motion coupled cluster is used in XAS calculations, the ground state is applied to the excitation operator, which excites or ionizes the electron. This causes a large orbital relaxation error, normally ~5 eV, which leads to the need for triple excitations in order to obtain accurate results. This dissertation introduces a coupled cluster method that uses transition potential reference orbitals to reduce the orbital relaxation error and help with error cancellation, called transition-potential coupled cluster (TP-CC), that is tested on our 14 small molecule data set. Then, the TP-CC fractional core orbital occupation number is optimized for a specific element and tested on the data set. The results of the optimized core orbital occupation number are utilized in nucleobase x-ray absorption K-edge spectra calculations and compared with experimental data. Another coupled cluster method is introduced to address the issue of orbital relaxation through the addition of triples excitation only in the core ionization potential. The similarity-transformed equation-of-motion coupled cluster (STEOM-CC) method is used, with the additional inclusion of core-valence separation (CVS) and correlation of triple excitations only within the calculation of core ionization energies, called CVS-STEOM-CCSD+cT. Our new method, CVS-STEOM-CCSD+cT is tested on our data set and compared to previously developed methods. Then, transition moments for CVS-STEOM-CCSD+cT excited states are implemented and tested on our data set. Lastly, tensor hypercontraction for open-shell systems is implemented and tested on molecules such as radicals, bond cleavages, and solvation shells, and the errors are compared to the errors obtained when using closed-shell tensor hypercontraction

Applications of electronic structure theory to problems in strong-field chemistry, inorganic chemistry, and nanomaterial systems

This dissertation covers research performed on applications of electronic structure theory to various fields of chemistry and is divided into eight chapters. Chapters 2 through 4 describe a series of related works which explore applications of excited state electronic structure methods to problems in strong field chemistry. Chapters 5, 6, and 7 discuss the application of electronic structure theory methods to solving problems in inorganic chemistry. Finally, Chapter 8 looks at an application of electronic structure theory to nanomaterials. Chapter 2 covers the modeling of electron dynamics of butadiene interacting with a short, intense laser pulse in the absence of ionization This chapter lays down the ground work for the following two chapters by examining the effects of basis set size and number of excited states included in the TD-CI simulation on the amount of population transferred from the ground state into the excited states by the interaction with an short, intense, non-resonant laser pulse. This chapter focuses mostly on TD-CI simulations using excited state energies and transition dipole matrices found by wavefunction based methods: TD-HF, TD-CIS, and TD-CIS(D). Chapter 3 expands on the work established in Chapter 2 by examining the excited state populations of butadiene using excitation energies and transition dipoles calculated by time-dependent density functional theory. Several DFT functionals are tested including GGA, meta-GGA, hybrid and long-range corrected functionals. The degree to which excited state energies and transition dipoles contribute to the final populations of the excited states is also examined. Chapter 4 wraps up the series by including ionization using a heuristic ionization model. This chapter examines the strong-field ionization of a series of linear polyenes of increasing length: ethylene, butadiene, hexatriene, and octatetraene. Also tested is the ionization dependence on parameters of the ionization model, basis set size, and number of states included in the simulation. Chapters 5-7 discuss collaborative works with members of the inorganic division of chemistry at Wayne State University. Chapter 5 describes a study on a chiral pentadenate ligand synthesized by the Kodanko group and the geometrical preference for a single isomer out of five possible isomers. Electronic structure theory indicates that the favored geometry is due to the chiral ligand, which prefers to be in a single conformation in metal complexes due to steric interactions. Chapter 6 covers a paddlewheel dinculear Cu(II) complex synthesized by the Winter group. This complex has the shortest Cu--Cu separation reported to date and electronic structure theory is used to explore the cause of this small separation. A simple model is proposed where the metal separation is governed by twisting of the ligand due to interligand π orbital interactions. Chapter 7 describes work done in collaboration with the Verani group, exploring the redox properties of some five-coordinate Fe(III) complexes. Chapter 8 sets out to develop an inexpensive model that can be used to optimize guest systems inside single walled nanotubes. The model takes advantage of the highly polarizable nature of nanotubes. The model is calibrated using a simple hydrogen bonded system and comparisons are made to test the reliability of the model

An assessment of different electronic structure approaches for modeling time-resolved x-ray absorption spectroscopy

We assess the performance of different protocols for simulating excited-state x-ray absorption spectra. We consider three different protocols based on equation-of-motion coupled-cluster singles and doubles, two of them combined with the maximum overlap method. The three protocols differ in the choice of a reference configuration used to compute target states. Maximum-overlap-method time-dependent density functional theory is also considered. The performance of the different approaches is illustrated using uracil, thymine, and acetylacetone as benchmark systems. The results provide guidance for selecting an electronic structure method for modeling time-resolved x-ray absorption spectroscopy

Development of nonorthogonal wavefunction theories and application to multistate reaction processes.

Many prominent areas of technological development rely on exploiting the photochemical response of molecules. An application of particular interest is the control of molecular switches through a combination of different external stimuli. However, despite significant advances in theoretical approaches and numerous cases of successful application of theory, simulating photochemical reactions remains a computational challenge. Theoretical methods for describing excited states can be broadly divided into single-reference response methods and multireference methods. Single reference methods provide reliable semiquantitative results for single excitations. However, these methods cannot describe double-excited states, systems with strongly correlated ground states, or regions of degeneracy on the potential energy surface. The alternative, multireference methods, can provide more accurate results. However, multireference methods require significant technical and chemical insight and become computationally costly as the system size increases. I will discuss my work applying newly developed and well-known methods for understanding multistate processes. I will highlight the limitations and extent of current methodologies that prevent researchers from studying larger and more complex systems. I will also discuss new methodological developments using spin projection, which seeks to overcome several problems of single reference excited state models. I will illustrate the motivation and its performance compared to more established theories. Despite its success, the new method cannot account for ‘multiple correlation mechanisms’. As a result, I will introduce how multiple correlation mechanisms can be exploited to perform nonorthogonal active space decomposition, along with applications and paths for future improvements

Combined Self-consistent-field And Spin-flip Tamm-dancoff Density Functional Approach To Potential Energy Surfaces For Photochemistry

We present a new approach to calculating potential energy surfaces for photochemical reactions by combining self-consistent-field calculations for single-reference ground and excited states with symmetry-corrected spin-flip Tamm-Dancoff approximation calculations for multireference electronic states. The method is illustrated by an application with the M05-2X exchange-correlation functional to cis-trans isomerization of the penta-2,4-dieniminium cation, which is a model (with three conjugated double bonds) of the protonated Schiff base of retinal. We find good agreement with multireference configuration interaction-plus-quadruples (MRCISD+Q) wave function calculations along three key paths in the strong-interaction region of the ground and first excited singlet states

Multireference approaches for excited states of molecules

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications

A Theoretical Perspective on the Photochemistry of Boron-Nitrogen Lewis Adducts

Boron-Nitrogen (B-N) Lewis adducts form a versatile family of compounds with numerous applications in functional molecules. Despite the growing interest in this family of compounds for optoelectronic applications, little is currently known about their photophysics and photochemistry. Even the electronic absorption spectrum of ammonia borane, the textbook example of a B-N Lewis adduct, is unavailable. Given the versatility of the light-induced processes exhibited by these molecules, we propose in this work a detailed theoretical study of the photochemistry and photophysics of simple B-N Lewis adducts. We used advanced techniques in computational photochemistry to identify and characterize the possible photochemical pathways followed by ammonia borane, and extended this knowledge to the substituted B-N Lewis adducts pyridine-borane and pyridine-boric acid. The photochemistry observed for this series of molecules allows us to extract qualitative rules to rationalize the light-induced behavior of more complex B-N containing molecules