Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L58: DFT and Beyond VIIFocus

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Sponsoring Units: DCP DCOMP DPOLY DCMP Chair: Alberto Vela, CINVESTAVIPN Room: Mile High Ballroom 3B 
Wednesday, March 4, 2020 8:00AM  8:36AM 
L58.00001: Quasiparticle Energies and Excitation Energies from Ground State DFT Calculations Invited Speaker: Weitao Yang The perspectives of fractional charges and fractional spins provide a clear analysis of the errors of commonly used density functional approximations (DFAs). These errors, the delocalization and static correlation error, of popular DFAs lead to diversified problems in presentday density functional theory calculations. To achieve a universal elimination of these two errors, we developed a localized orbital scaling correction (LOSC) framework. The LOSC–DFAs lead to systematically improved results, including the dissociation of ionic species, single bonds, multiple bonds without breaking space or spin symmetry, the band gaps of molecules and polymer chains, the energy and density changes upon electron addition and removal, and photoemission spectra. Comparison of experimental quasiparticle energies for many finite systems with calculations from the GW Green function approach and LOSC shows that LOSC orbital energies achieve slightly better accuracy than the GW calculations with little dependence on the semilocal DFA, supporting the use of LOSC DFA orbital energies to predict quasiparticle energies. This leads the development of the Quasiparticle Energy DFT (QEDFT) approach to the calculations of excitation energies of the Nelectron systems from the ground state DFA calculations of the (N  1)electron systems. Results show good performance of QEDFT for valence excitations with commonly used DFAs with or without LOSC, for Rydberg states only with the use of LOSCDFA, and the accurate description of conical interactions. This highlights a new and simplest pathway to describe excited states. 
Wednesday, March 4, 2020 8:36AM  8:48AM 
L58.00002: Selfconsistent firstprinciples method for extended Hubbard interactions SangHoon Lee, YoungWoo Son We present an efficient selfconsistent firstprinciples computational approach that extends the density functional theory plus onsite interaction (DFT+U) method further to include intersite Hubbard Coulomb interaction (V). The extended DFT+U +V method is suitable to calculate electronic structures of periodic systems with various interactions with disparate spatial ranges. It has been known that typical computational methods for DFT+U and DFT+U+V require additional set of computations to obtain U and (or) V. In this presentation, we suggest a scheme to compute the Hubbard parameters selfconsistently and ab initio without additional computations for a set of parameters of U and V. A few examples of calculations of energy bands for semiconductors and insulators will also be presented. 
Wednesday, March 4, 2020 8:48AM  9:00AM 
L58.00003: Selfconsistent DFT+U+V study of oxygen vacancies in SrTiO_{3} Chiara Ricca, Iurii Timrov, Matteo Cococcioni, Nicola Marzari, Ulrich Aschauer DFT calculations of defects in transitionmetal oxides constitute a challenging task, often requiring advanced methods to ensure a reasonable description of the electronic structure and large supercells to mimic the dilute defect concentration. Several contradicting DFT results were reported for oxygen vacancies (V_{O}) in SrTiO_{3} (STO) and were often related to the peculiar properties of STO, which is a d^{0} transition metal oxide with mixed ioniccovalent bonding. Here, for the first time, we apply the extended Hubbard DFT+U+V approach, including onsite (Ti3d) as well as intersite (Ti3d and O2p) electronic interactions, to study oxygendeficient STO with Hubbard U and V parameters computed selfconsistently via densityfunctional perturbation theory. The negligible additional cost of DFT+U+V compared to standard DFT enables the treatment of large supercells, yet the obtained structural and electronic properties agree well with hybridfunctional calculations and experiments. As such, DFT+U+V results in a bandgap and crystal field splitting for STO in good agreement with experiments. In turn, the description of the electronic properties of V_{O }is improved, with formation energies much less dependent on the cell size compared to DFT+U and in excellent agreement with experiments. 
Wednesday, March 4, 2020 9:00AM  9:12AM 
L58.00004: Spectral function database for correlated materials using beyondDFT methods Subhasish Mandal, Kristjan Haule, Karin M Rabe, David Vanderbilt While DFT or DFT+U methods give quite accurate results for structural parameters in most materials, correct predictions of excitedstate properties, even at a qualitative level, and other properties of correlated materials, usually require beyondDFT methods. The existing materials databases, constructed in the spirit of the Materials Genome Initiative, are built almost exclusively by the DFT method and are thus very often making incorrect predictions for correlated materials. Various beyond DFT methods, such as metaGGA, hybrid functionals, GW approximation, or dynamical meanfield theory have been developed to describe the electronic structure of correlated materials, but it is unclear how accurate these methods are when applied to a given strongly correlated solid. It is thus of pressing interest to compare their accuracy as applied to different categories of materials, and at the same time, to build the database of beyondDFT calculations. We discuss a systematic study of these methods on a few training sets of correlated materials such as binary transitionmetal oxides, Fepnictides & chalcogenides, and transitionmetal dichalcogenides, and we compare theoretical predictions with experimental photoemission data, where available. 
Wednesday, March 4, 2020 9:12AM  9:24AM 
L58.00005: Assessment of excitedstate molecular geometries with optimallytuned rangeseparated hybrid functionals Bernhard Kretz, David Alexander Egger Computational modelling of photochemical processes (e.g., for photocatalysis) requires accurate descriptions of excitedstate structural dynamics of the involved molecules. Often, the starting point of such investigations are geometries optimized for the lowestlying excited state, as obtained in timedependent density functional theory (TDDFT) or highlevel wavefunction methods. While calculations based on TDDFT are computationally very efficient, they often do not reach the accuracy of computationally more expensive wavefunction methods[1]. However, the recently developed class of optimallytuned rangeseparated hybrid (OTRSH) functionals promises to reduce the gap in accuracy[2]. 
Wednesday, March 4, 2020 9:24AM  9:36AM 
L58.00006: KohnSham Density Functional Theory with Complex, SpinRestricted Orbitals: Accessing a New Class of Densities without the Symmetry Dilemma Joonho Lee, Martin P HeadGordon We show that using complex, spinrestricted orbitals in KohnSham (KS) density functional theory allows one to access a new class of densities that is not accessible by either spinrestricted (RKS) or spinunrestricted (UKS) orbitals [1]. We further show that the real part of a complex RKS (CRKS) density matrix can be nonidempotent when the imaginary part of the density matrix is not zero. Using CRKS orbitals shows significant improvements in the tripletsinglet gaps of a benchmark set, called TS12, for wellestablished, widely used density functionals. Moreover, it was shown that RKS and UKS yield qualitatively wrong charge densities and spin densities, respectively, leading to worse energetics. We demonstrate that representative modern density functionals show surprisingly no improvement even with a qualitatively more accurate density from CRKS orbitals. To this end, our work not only provides a way to escape the symmetry dilemma whenever there exists a CRKS solution, but also suggests a new route to design better approximate density functionals. 
Wednesday, March 4, 2020 9:36AM  9:48AM 
L58.00007: "Implementation of imaginary time dependent density functional theory to periodic and noncollinear systems" John McFarland Recently an alternative to the SCF method for calculating the DFT electronic ground state was proposed that evolves the DFT wave function in imaginary time [1]. One benefit of this new method is that the DFT wave function is guaranteed to lower its energy and eventually reach the ground state with a sufficiently small time step. This avoids the problems in SCF that hinder convergence such as charge sloshing, which can necessitate fractional level filling. We extend imaginary time dependent density functional theory to periodic systems using a modification of the DFT package Quantum ESPRESSO, with the option of noncollinear and DFT+U calculations. We discuss some of the technical aspects of this as well as example systems which converge with this modification but have difficulty with standard implementations of SCF. 
Wednesday, March 4, 2020 9:48AM  10:00AM 
L58.00008: PyProcar: A Python library for electronic structure pre/postprocessing Uthpala Herath, Pedram Tavadze, Xu He, Eric Bousquet, Sobhit Singh, Francisco Munoz, Aldo H Romero We present PyProcar, a Python package providing graphical representations for electronic structure calculations including band structures and Fermi surfaces as a function of atomic and/or s, p, d, f – orbital projected wavefunctions. This is compatible with DFT codes which output band and projection information in the PROCAR format, as done by the VASP and ABINIT codes. PyProcar is particularly suitable for studying atomic effects into the band structure, Fermi surface and spin texture. Aside from spin, orbital and atom projected band structures and Fermi surfaces, PyProcar plots Fermi surfaces which map colors to properties such as the electron velocity, electronphonon mean path and effective mass. Another existing feature refers to the band unfolding of supercell calculations into predefined unit cells. PyProcar can be conveniently used in either a standalone command line mode or a library mode easily accessible through the Python packaging index (pip) and performs tasks with simple commands. 
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