Publications

Mechanism of water oxidation catalyzed by cobalt-intercalated layered MnO 2 : confinement and intercalant local ordering

Erratum: Simple self-interaction correction to random-phase-approximation-like correlation energies [Phys. Rev. A 100, 022515 (2019)]

The random phase approximation (RPA) is exact for the exchange energy of a many-electron ground state, but RPA makes the correlation energy too negative by about 0.5 eV/electron. That large short-range error, which tends to cancel out of iso-electronic energy differences, is largely corrected by an exchange-correlation kernel, or (as in RPA+) by an additive local or semilocal correction. RPA+ is by construction exact for the homogeneous electron gas, and it is also accurate for the jellium surface. RPA+ often gives realistic total energies for atoms or solids in which spin-polarization corrections are absent or small. RPA and RPA+ also yield realistic singlet binding energy curves for H2 and N2, and thus RPA+ yields correct total energies even for spin-unpolarized atoms with fractional spins and strong correlation, as in stretched H2 or N2. However, RPA and RPA+ can be very wrong for spin-polarized one-electron systems (especially for stretched H2+), and also for the spin-polarization energies of atoms. The spin-polarization energy is often a small part of the total energy of an atom, but important for ionization energies, electron affinities, and the atomization energies of molecules. Here we propose a computationally efficient generalized RPA+ (gRPA+) that changes RPA+ only for spin-polarized systems by making gRPA+ exact for all one-electron densities, in the same simple semilocal way that the correlation energy densities of many meta-generalized gradent approximations are made self-correlation free. By construction, gRPA+ does not degrade the exact RPA+ description of jellium. gRPA+ is found to greatly improve upon RPA and RPA+ for the ionization energies and electron affinities of light atoms. Many versions of RPA with an approximate exchange-correlation kernel fail to be exact for all one-electron densities, and they can also be self-interaction corrected in this way.

On the local density approximation for Breit interaction

The large relative error of the local density approximation for the transverse-photon-electron or Breit energy in atoms and solids is shown to arise from spurious self-interaction of the 1s electrons. Orbital self-interaction correction dramatically reduces this error. On the other hand, the large relative error of the local density approximation for the Breit contribution to the 2s electron removal energy is almost unchanged by self-interaction correction.

Sensitivity of the electronic and magnetic structures of cuprate superconductors to density functional approximations

Abstract We discuss the crystal, electronic, and magnetic structures of La 2− x Sr x CuO 4 (LSCO) for x = 0.0 and x = 0.25 employing 13 density functional approximations, representing the local, semi-local, and hybrid exchange-correlation approximations within the Perdew–Schmidt hierarchy. The meta-generalized gradient approximation (meta-GGA) class of functionals is found to perform well in capturing the key properties of LSCO, a prototypical high-temperature cuprate superconductor. In contrast, the localspin-density approximation, GGA, and the hybrid density functional fail to capture the metal-insulator transition under doping.

Short-range correlation in the uniform electron gas: Extended Overhauser model

We use the two-electron wavefunctions (geminals) and the simple screened Coulomb potential proposed by Overhauser [Can. J. Phys. 73, 683 (1995)] to compute the pair-distribution function for a uniform electron gas. We find excellent agreement with Quantum Monte Carlo simulations in the short-range region, for a wide range of electron densities. We are thus able to estimate the value of the second-order coefficient of the small interelectronic-distance expansion of the pair-distribution function. The results are generalized to the partially polarized gas.

Self-expansion and compression of charged clusters of stabilized jellium

In a positively charged metallic cluster, surface tension tends to enhance the ionic density with respect to its bulk value, while surface-charge repulsion tends to reduce it. Using the stabilized jellium model, we examine the self-expansion and compression of positively charged clusters of simply metals. Quantal results from the Kohn-Sham equations using the local density approximation are compared with continuous results from the liquid drop model. The positive background is constrained to a spherical shape. Numerical results for the equilibrium radius and the elastic stiffness are presented for singly and doubly positively charged aluminum, sodium, and cesium clusters of 1–20 atoms. Self-expansion occurs for small charged clusters of sodium and cesium, but not of aluminum. The effect of the expansion or compression on the ionization energies is analyzed. For Al6, we also consider net charges greater than 2+. The results of the stabilized jellium model for self-compression are compared with those of other models, including the SAPS (spherical averaged pseudopotential model). © 1996 John Wiley & Sons, Inc.

Nonlocality of the density functional for exchange and correlation: Physical origins and chemical consequences

Gradient corrections to the local spin density approximation for the exchange-correlation energy Exc are increasingly useful in quantum chemistry and solid state physics. We present elementary physical arguments which explain the qualitative dependencies of the exchange and correlation energies upon the local density, local spin polarization, and reduced density gradient. The nearly local behavior of the generalized gradient approximation for Exc at valence-electron densities, due to strong cancellation between the nonlocalities of exchange and correlation, is shared by the exact linear response of the uniform electron gas. We further test and develop our rationale for the chemical and solid-state consequences of gradient corrections. We also partially explain the “conjointness” between the exchange energy and the noninteracting kinetic energy, whose generalized gradient approximation is tested here. An appendix presents the full expression for the gradient-corrected correlation potential.

Density of states and spin susceptibility of a finite metal: Normal and giant surface effects

The size effect on the electronic density of states and spin susceptibility $\ensuremath{\chi}$ of small metal particles and thin films is evaluated to order (area/volume) within the spin-density functional formalism. Conditions are given under which the size effect arises only in the surface region. Size-effect parameters and magnetization profiles, as well as surface energies and work functions, are calculated self-consistently for the jellium surface using the random-phase approximation and the local spin-density approximation (LSDA) for exchange and correlation. The jellium surface is also used to test two approximations for $\ensuremath{\chi}$ which require only spin-unpolarized calculations. One of them, the Vosko-Perdew (VP) approximation, is found to agree closely with the full spin-polarized calculation within the LSDA. The VP theory, which gives lower bounds for the bulk and surface susceptibilities and does not necessarily require the LSDA, is used to discuss the possibility of a giant surface effect, i.e., a giant enhancement of the surface susceptibility, in any metal that is nearly ferromagnetic. The giant surface effect is illustrated by an LSDA calculation for a low-density jellium, and by two other instructive models.

Sensitivity of the electronic and magnetic structures of high-temperature cuprate superconductors to exchange-correlation density functionals.

We discuss the crystal, electronic, and magnetic structures of $\mathrm{La_{2-x}Sr_{x}CuO_{4}}$ for $x = 0.0$ and $x = 0.25$ employing 13 density functionals, representing the local, semi-local, and hybrid exchange-correlation approximations within the Perdew-Schmidt hierarchy of functionals. We find that the meta-generalized gradient approximation (meta-GGA) class of functionals perform well in capturing the key properties of LSCO, which is a prototypical high-temperature cuprate superconductor. In contrast, the local-spin-density approximation (LSDA), GGA, and the hybrid density functional fail to capture the metal-insulator transition (MIT) under doping.

Reverse-Engineering the Exchange-Correlation hole for the SCAN and r 2 SCAN Functional

DFT Exchange: Sharing Perspectives on the Workhorse of Quantum Chemistry and Materials Science

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 300 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 776 entries, the paper represents a broad snapshot of DFT, anno 2022.

Evolution of Antiferromagnetism with Hole Doping in HgBa 2 Ca 2 Cu 3 O 8 : A Parameter Free Perspective

Investigation and analysis of the radiation protection status of radiation workers during the peri-pregnancy period

Radiation protection measures were more effectively implemented in public hospitals than in other institutions, underscoring the need for standardized policies across all institutions. Female practitioners exhibited heightened concerns about radiation exposure of the fetus and infant, particularly during pregnancy and lactation. Strengthening policies and workplace adjustments are critical to mitigating occupational risks and safeguarding maternal and child health.

The SCAN Meta-GGA : An Efficient Universal Non-Empirical Semi-local Density Functional

Accurate DFT simulation of complex functional materials: Synergistic enhancements achieved by SCAN meta-GGA

Complex functional materials are characterized by intricate and competing bond orders, making them an excellent platform for evaluating the newly developed strongly constrained and appropriately normed (SCAN) density functional. In this study, we explore the effectiveness of SCAN in simulating the electronic properties of displacive ferroelectrics (BaTiO3 and PbTiO3) and magnetoelectric multiferroics (BiFeO3 and YMnO3), which encompass a broad spectrum of bonding characteristics. Due to a significant reduction in self-interaction error, SCAN manifests its improvements over the Perdew-Burke-Ernzerhof (PBE) method in three aspects: SCAN predicts more accurate ionicity, produces more compact orbitals, and better captures d-orbital anisotropy. Particularly, these synergistic enhancements lead to notable phenomena in calculating the bandgap of YMnO3: while the PBE+U simulation may suggest a strong correlation appearance attributed to high Hubbard-like U values (∼5 eV), the value is dramatically lower (∼1 eV) in the SCAN+U method. Furthermore, we provide an intuitive analysis of SCAN's operational principles by examining the complex electron densities involved. These insights are theoretically intriguing and have practical implications, potentially encouraging wider adoption of SCAN in the computational modeling of complex functional materials.

Van der Waals Coefficients for Nanostructures: Fullerenes Defy Conventional Wisdom

The van der Waals coefficients between quasispherical nanostructures can be modeled accurately and analytically by those of classical solid spheres (for nanoclusters) or spherical shells (for fullerenes) of uniform valence electron density, with the true static dipole polarizability. Here, we derive analytically and confirm numerically from this model the size dependencies of the van der Waals coefficients of all orders, showing, for example, that the asymptotic dependence for C(6) is the expected n(2) for pairs of nanoclusters A(n)-A(n), each containing n atoms, but n(2.75) for pairs of single-walled fullerenes C(n)-C(n). Large fullerenes are argued to have much larger polarizabilities and dispersion coefficients than those predicted by either the standard atom pair-potential model or widely used nonlocal van der Waals correlation energy functionals.

Comments on the metal surface from a simple analytic model

Very accurate surface energies and work functions for a semi-infinite jellium can be calculated from a simple and transparent analytic model. The model is a modified version of one proposed by Smith and refined by Ma and Sahni which employs the gradient expansion for the kinetic energy and a one-parameter trial density profile; the modifications are use of a more-realistic profile and a less-profile-sensitive expression for the work function. This model is used to explore the qualitative effects of exchange and correlation on the metal surface properties, as well as the significant quantitative effects of different choices for the bulk correlation energies ${\ensuremath{\epsilon}}_{c}(n)$ which serve as input to the local-density approximation.

Hartree-Fock density functional theory works through error cancellation for the interaction energies of halogen and chalcogen bonded complexes

Semi-local functionals are often more accurate for energies when evaluated on the Hartree-Fock (HF) density than on their own self-consistent densities. HF densities are known to greatly reduce or overcorrect the charge-delocalization error associated with semi-local functionals. Over the last decade, this Hartree-Fock density functional theory (HF-DFT) has been systematized as density corrected DFT (DC-DFT), demonstrating remarkable success: improved chemical barrier heights, near chemical accuracy in water cluster binding energies, highly accurate interaction energies for halogenand chalcogen bonded systems, and more. For reaction barriers and water clusters, some of us found earlier that HFDFT works, not because the HF density is accurate but because of cancellation of negative functional-driven error (FE) by positive density-driven error (DE). In this work, we present evidence that interaction energy errors in halogen and chalcogen bonded molecular complexes in the B30 data set are not primarily driven by density errors, and that the success of HF-DFT for these weakly bonded molecular complexes results from a similar cancellation of FE by DE. Our benchmark Kohn-Sham inversion of the coupled cluster densities for NH3 · · ·ClF, Cl− · · ·SF2 and Cl− · · ·SCF2 presents strong evidence for this error cancellation. For most of the complexes, we employ proxies for the electron transfer in the exact density: the LCωPBE long-range-corrected hybrid and the r2SCAN50 global hybrid. We further investigate several self-interaction correction (SIC) methods for these weakly bonded systems, finding significant improvement from FLOSIC. In the conclusions section, we point out the common feature in our present and previous work: Long bonds can lead to non-negligible functional-driven self-interaction error of the energy from otherwise-accurate semi-local functionals in transition states, water clusters, and halogen or chalcogen bonds.