Kohn-Sham density functional theory (DFT) is a widely-used electronic structure theory for materials as well as molecules. DFT is needed especially for large systems, ab initio molecular dynamics, and high-throughput searches for functional materials. DFT's accuracy and computational efficiency are limited by the approximation to its exchange-correlation energy. Currently, the local density approximation (LDA) and generalized gradient approximations (GGAs) dominate materials computation mainly due to their efficiency. We show here that the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-GGA improves significantly over LDA and the standard Perdew-Burke-Ernzerhof GGA for geometries and energies of diversely-bonded materials (including covalent, metallic, ionic, hydrogen, and van der Waals bonds) at comparable efficiency. Thus SCAN may be useful even for soft matter. Often SCAN matches or improves upon the accuracy of a computationally expensive hybrid functional, at almost-GGA cost. SCAN is therefore expected to have a broad impact on materials science.
The accuracy and computational efficiency of the widely used Kohn-Sham density functional theory (DFT) are limited by the approximation to its exchange-correlation energy Exc. The earliest local density approximation (LDA) overestimates the strengths of all bonds near equilibrium (even the vdW bonds). By adding the electron density gradient to model Exc, generalized gradient approximations (GGAs) generally soften the bonds to give robust and overall more accurate descriptions, except for the vdW interaction which is largely lost. Further improvement for covalent, ionic, and hydrogen bonds can be obtained by the computationally more expensive hybrid GGAs, which mix GGAs with the nonlocal exact exchange. Meta-GGAs are still semilocal in computation and thus efficient. Compared to GGAs, they add the kinetic energy density that enables them to recognize and accordingly treat different bonds, which no LDA or GGA can [2]. In this talk, I will present an advance in DFT, the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-GGA [1]. SCAN predicts accurate geometries and energies of diversely-bonded molecules and materials (including covalent, metallic, ionic, hydrogen, and van der Waals bonds), significantly improving over its predecessors, the GGAs that dominate materials computation, at comparable efficiency [2]. SCAN’s excellent performance on formation enthalpies and ground structure predictions, critical for materials discovery and design, will be highlighted [3]. I will further explain how SCAN was constructed [1], why it can improve over GGAs [2], and where it should fail [4]. At the end, efforts to improve SCAN via nonlocal corrections will be discussed. [1] J. Sun, A. Ruzsinszky, and J.P. Perdew, Strongly constrained and appropriately normed semilocal density functional, PRL 115 , 036402 (2015). [2] J. Sun, R.C. Remsing, Y. Zhang, Z. Sun, A. Ruzsinszky, H. Peng, Z. Yang, A. Paul, U. Waghmare, X. Wu, M.L. Klein, and J.P. Perdew, Accurate First-principles structures and energies of diversely-bonded systems from an efficient density functional, Nat. Chem. 8 , 831 (2016). [3] Y. Zhang, D.A. Kitchaev, J. Yang, T. Chen, S.T. Dacek, R.A. Sarmiento-Perez, M.A.L. Margues, H. Peng, G. Ceder, J.P. Perdew, and J. Sun, Efficient first-principles prediction of solid stability: Towards chemical accuracy, NPJ Computational Materials 4 , 9 (2018). [4] H. Peng, Z. Yang, J.P. Perdew, and J. Sun, Versatile van der Waals density functional based on a meta-generalized gradient approximation, PRX 6 , 041005 (2016).
Electrodeposition of an amorphous cobalt catalyst layer over a high-surface-area alpha-Fe(2)O(3) photoanode causes a more than 350 mV cathodic shift in the onset potential for photoelectrochemical water oxidation using this anode and simulated solar irradiation. The catalyst layer is shown to deposit conformally onto the mesostructured alpha-Fe(2)O(3), leading to a large contact area at the interface between the two halves of the composite photoanode. Photoelectrochemical measurements show that the photocurrent generated from this composite photoanode still derives from alpha-Fe(2)O(3) excitation but is now accessible at an external bias several hundred millivolts below what is typically required for alpha-Fe(2)O(3) photoanodes alone, indicating a reduced external bias would be needed to drive overall water splitting. These results demonstrate modification of this prototypical photoanode material with a conformal layer of a competent electrocatalyst to separate the tasks of photon absorption and redox catalysis, a strategy that may have important and general ramifications for solar photoelectrochemical hydrogen generation.
While the exact total energy of a separated open system varies linearly as a function of average electron number between adjacent integers, the energy predicted by semilocal density-functional approximations is concave up and the exact-exchange-only or Hartree-Fock energy is concave down. As a result, semilocal density functionals fail for separated open systems of fluctuating electron number, as in stretched molecular ions $\mathrm{A}_{2}{}^{+}$ and in solid transition-metal oxides. We develop an exact-exchange theory and an exchange-hole sum rule that explain these failures and we propose a way to correct them via a local hybrid functional.
We present a self-consistent calculation of the ground-state properties of simple metallic planar surfaces using density-functional theory with nonlocal exchange-correlation effects included. Our calculational scheme closely follows the classic work of Lang and Kohn for the jellium model, except that the exchange-correlation energy and potential within the local-density approximation (LDA) are replaced by the corresponding nonlocal functionals of Langreth and Mehl (LM). We also include the discrete-lattice effects, following the variational scheme of Perdew and Monnier. The physical properties considered include the one-electron effective potential, charge-density profile, surface energy, and work function. For each of the most densely packed surfaces of seven simple metals with fcc or bcc structure, we find that, when compared with results in the LDA, the Friedel oscillation in the electron density near the surface is systematically depressed as a result of positive LM contributions to the effective potential at places where the LDA density oscillation is peaked. We find a systematic increase in surface energies, with bigger increases for higher-density metals. Increases are also found in the work functions. Our results and those from other density-functional calculations are compared with results from variational treatments of the ground-state wave function, and with experiment. We also comment upon the Fermi hypernetted-chain jellium surface energies and work functions.
Received 4 January 2018DOI:https://doi.org/10.1103/PhysRevB.98.079903©2018 American Physical SocietyPhysics Subject Headings (PhySH)TechniquesDensity functional theory developmentCondensed Matter, Materials & Applied Physics
In recent work, generalized gradient approximations (GGAs) have been constructed from the energy density of the Airy gas for exchange but not for correlation. We report the random-phase approximation (RPA) conventional correlation energy density of the Airy gas, the simplest edge electron gas, in which the auxiliary noninteracting electrons experience a linear potential. By fitting the Airy-gas RPA exchange-correlation energy density and making an accurate short-range correction to RPA, we propose a simple beyond RPA GGA density functional (``$\text{ARPA}+$'') for the exchange-correlation energy. Our functional, tested for jellium surfaces, atoms, molecules, and solids, improves mildly over the local spin-density approximation for atomization energies and lattice constants without much worsening the already good surface exchange-correlation energies.
Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of “bottom‐up” growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one‐dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.
Pair-distribution function and its coupling
