Novel approaches to spectral properties of correlated electron materials: From generalized Kohn-Sham theory to screened exchange dynamical mean field theory

The most intriguing properties of emergent materials are typically consequences of highly correlated quantum states of their electronic degrees of freedom. Describing those materials from first principles remains a challenge for modern condensed matter theory. Here, we review, apply and discuss novel approaches to spectral properties of correlated electron materials, assessing current day predictive capabilities of electronic structure calculations. In particular, we focus on the recent Screened Exchange Dynamical Mean-Field Theory scheme and its relation to generalized Kohn-Sham theory. These concepts are illustrated on the transition metal pnictide BaCo$_2$As$_2$ and elemental zinc and cadmium.Comment: Accepted for publication in the Journal of the Physical Society of Japa

First-principles study of spontaneous polarization in multiferroic BiFeO3_3

The ground-state structural and electronic properties of ferroelectric BiFeO$_3$ are calculated using density functional theory within the local spin-density approximation and the LSDA+U method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of 90-100 $\mu$C/cm$^2$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.Comment: (9 pages, 5 figures, 5 tables

Modern Multireference Electronic Structure Theory

Chemical systems that contain transition metals or are far from their equilibrium geometries are often impossible to describe with a single electronic configuration. These problems require special methods, which we refer to as multireference, that explicitly treat more than one configuration. In this thesis, I describe the development and application of an efficient configuration interaction (CI)solver called the Heat-bath CI (HCI) algorithm. In Chapter 2, I study the use of HCI in complete active space self-consistent field (CASSCF) calculations. Since the perturbatively corrected HCIenergy is not variational, we use a Lagrangian formalism to simplify the orbital gradient expressions. We apply this new method, called HCISCF, to butadiene, pentacene, and Fe(porphyrin). HCISCFcorrectly predicts the multiplicity of the Fe(porphyrin) ground state, which many multireference methods cannot. Then in Chapter 3, I present the gradients of HCISCF wave functions and their use in geometry optimizations. We show that the gradients converge smoothly to the exact answers as a function of the HCI parameter e1 and that gradients of inaccurate variational wave functions can be improved by including the perturbative correction. In addition to the well studied species ozone and p-benzyne, we use these gradients to find the equilibrium structure for the iron 2,6-bis[1-(2,6-dimethylphenyl-imino)ethyl]pyridine complex in the singlet and triplet states. In Chapter 4, I explore applications of CASSCF and multireference perturbation theory to calculate the electronic excitation of free-base Protopophyrin IX. This species was recently studied by our collaborators in the Weber group and our theoretical simulations agree well with their experimental results. Finally,in Chapter 5, I conclude with a discussion of our collaboration with the George group where we use statistical learning models to predict the outcome of atomic layer etching reactions.</p

Modern Electronic Structure Theory: The Search for Chemical Accuracy

Electronic structure theory has progressed significantly within the last few decades, venturing far from the early days of the Hartree-Fock self-consistent field method. Modern electronic structure theory focuses on compound methods, which operate under the idea that we can take a lower level of theory computation (typically, a result from Hartree-Fock, Configuration Interaction, Coupled Cluster or Moller-Plesset perturbation theory) and add in higher level of the theory corrections such as extrapolations to the infinite basis set limit, as well as, relativistic effects. Using the Gaussian-n, Complete Basis Set and Weizmann compound methods, we were able to provide theoretical evidence to justify the claim that the mechanism for the isomerization process of perfluoro-2-azapropene was through either a nitrogen inversion or rotational mechanism. Following the previous study was the realization that what is predicted to be the most accurate compound method (the Weizmann method) doesn’t yield the most accurate result, led us to ask the question “Is there a compound method available that’s both computationally feasible on a workstation computer, as well as, able to produce the best results regardless of the molecule or process being studied?”. What we found was that the Weizmann-2 method is computationally feasible on a workstation computer, as well as, claims to produce chemically accurate results (results within 1 kcal mole-1) from there experimental values for all molecules and processes. However, the Weizmann-2 method has only been tested against thermochemical data with little to no work being done with any kinetic parameter. These realizations sparked our interest to verify the validity of this claim by testing the accuracy of the Weizmann-2 method against a kinetic parameter such as a barrier height. The results of the Weizmann-2 investigation were then used to develop a modification to the Weizmann-2 method which was able to produce chemically accurate barrier heights for all of the well-behaved molecules studied

The Chronus Quantum software package

The Chronus Quantum (ChronusQ) software package is an open source (under the GNU General Public License v2) software infrastructure which targets the solution of challenging problems that arise in ab initio electronic structure theory. Special emphasis is placed on the consistent treatment of time dependence and spin in the electronic wave function, as well as the inclusion of relativistic effects in said treatments. In addition, ChronusQ provides support for the inclusion of uniform finite magnetic fields as external perturbations through the use of gauge-including atomic orbitals. ChronusQ is a parallel electronic structure code written in modern C++ which utilizes both message passing implementation and shared memory (OpenMP) parallelism. In addition to the examination of the current state of code base itself, a discussion regarding ongoing developments and developer contributions will also be provided. This article is categorized under: Software &gt; Quantum Chemistry Electronic Structure Theory &gt; Ab Initio Electronic Structure Methods Electronic Structure Theory &gt; Density Functional Theory

LSDA+U approximation-based analysis of the electronic estructure of CeFeGe3

We perform ab initio electronic structure calculations of the intermetallic compound CeFeGe3 by means of the Tight Binding Linear Muffin-Tin Orbitals-Atomic Sphere Approximation (TB-LMTO-ASA) within the Local Spin Density Approximation containing the so-called Hubbard correction term (LSDA+U^SIC), using the Sttutgart's TB (Tight Binding)-LMTO-ASA code in the framework of the Density Funcional Theory (DFT).Comment: 12 pages 8 figures, submitted to Int. J. Modern Phys.