An intense, positive electric field applied normal to a metal surface can displace or even strip away the surface layer of atoms. These effects are studied for a jellium model (${r}_{s}$=4.20 a.u., surface layer thickness d=5.65 a.u.) via fully self-consistent calculations within the local-density approximation for exchange and correlation. From plots of surface energy versus displacement for several fields of interest, the critical field ${F}_{c}$ required to evaporate the rigid surface layer is found (1.8 V/A\r{}) and compared with the prediction (1.7 V/A\r{}) of a simple semiempirical formula based upon universal binding-energy curves. The calculations also reveal information about electronic-charge redistribution, electronic resonances which develop with increasing separation of the surface layer from the bulk, and various components of the surface-layer--bulk binding force. The jellium surface is compared with the real-metal surface Na(110) and with Al(111), which was investigated in earlier semi-self-consistent work.
The oxygen evolution reaction (OER) from oxidation of water or hydroxide to dioxygen remains the most challenging part of solar water splitting. Inexpensive, robust, efficient catalysts for chemical and electrochemical OER are sought. We have been exploring the mineral phase birnessite (layered KnMnO2) for this reaction, as it shows significant variability in its activity based
Van der Waals interactions are ubiquitous in different materials yet not always described properly by current theories. Now, researchers have determined how to accurately and efficiently treat long-range Van der Waals interactions together with other chemical bonds, new findings that are important for studies of layered materials.
The SCAN (strongly constrained and appropriately normed) meta-generalized gradient approximation (meta-GGA), which satisfies all 17 exact constraints that a meta-GGA can satisfy, accurately describes equilibrium bonds that are normally correlated. With symmetry breaking, it also accurately describes some sd equilibrium bonds that are strongly correlated. While sp equilibrium bonds are nearly always normally correlated, the C<sub>2</sub> singlet ground state is known from correlated wave function theory to be a rare case of strong correlation in an sp equilibrium bond. Earlier work that calculated atomization energies of the molecular sequence B<sub>2</sub>, C<sub>2</sub>, O<sub>2</sub>, and F<sub>2</sub> in the local spin density approximation (LSDA), the Perdew-Burke-Ernzerhof (PBE) GGA, and the SCAN meta-GGA, without symmetry breaking in the molecule, found that only SCAN was accurate enough to reveal an anomalous under-binding for C<sub>2</sub>. This work shows that spin symmetry breaking in singlet C<sub>2</sub>, which involves the appearance of net up- and down-spin densities on opposite sides (not ends) of the bond, corrects that underbinding, with a small SCAN atomization-energy error more like that of the other three molecules, suggesting that symmetry breaking with an advanced density functional might reliably describe strong correlation. This article also discusses some general aspects of symmetry breaking and the insights into strong correlation that symmetry breaking can bring. The normally correlated low-lying triplet excited state has the right vertical excitation energy in SCAN but not in LSDA or PBE, where the triplet is a false ground state. Fractional occupation numbers are found only for the symmetry-unbroken singlet and only in LSDA and PBE GGA.
We present a new paradigm for the design of exchange-correlation functionals in density-functional theory. Electron pairs are correlated explicitly by means of the recently developed second order Bethe-Goldstone equation (BGE2) approach. Here we propose a screened BGE2 (sBGE2) variant that efficiently regulates the coupling of a given electron pair. sBGE2 correctly dissociates H_{2} and H_{2}^{+}, a problem that has been regarded as a great challenge in density-functional theory for a long time. The sBGE2 functional is then taken as a building block for an orbital-dependent functional, termed ZRPS, which is a natural extension of the PBE0 hybrid functional. While worsening the good performance of sBGE2 in H_{2} and H_{2}^{+}, ZRPS yields a remarkable and consistent improvement over other density functionals across various chemical environments from weak to strong correlation.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The Hohenberg-Kohn theorem is extended to fractional electron number $N$, for an isolated open system described by a statistical mixture. The curve of lowest average energy ${E}_{N}$ versus $N$ is found to be a series of straight line segments with slope discontinuities at integral $N$. As $N$ increases through an integer $M$, the chemical potential and the highest occupied Kohn-Sham orbital energy both jump from ${E}_{M}\ensuremath{-}{E}_{M\ensuremath{-}1}$ to ${E}_{M+1}\ensuremath{-}{E}_{M}$. The exchange-correlation potential $\frac{\ensuremath{\delta}{E}_{\mathrm{xc}}}{\ensuremath{\delta}n(\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}})}$ jumps by the same constant, and $\frac{{\mathrm{lim}}_{r\ensuremath{\rightarrow}\ensuremath{\infty}}\ensuremath{\delta}{E}_{\mathrm{xc}}}{\ensuremath{\delta}n(\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}})}>~0$.
Linear ${\mathrm{H}}_{2n}$ chains of ${\mathrm{H}}_{2}$ units are experimentally unrealizable but simple and widely studied exemplars of the response coefficients (polarizabilities and hyperpolarizabilities) of polymer chains in uniform longitudinal electric fields. They show two surprising features: (1) Their response coefficients, unlike those of bulk solids, show a strong nonlinear dependence upon length or $n$. (2) Their response coefficients are seriously too large when computed with standard local or semilocal density functionals, but are much more correct when computed with self-interaction-free approaches (including Hartree-Fock). We propose a simple charge-transfer model which explains both of these effects in analytic terms. In this model, charge is transferred between ${\mathrm{H}}_{2}$ units paired up at equal distances from but on opposite sides of the chain center. All symmetric pairs of ${\mathrm{H}}_{2}$ units, not just the one for the chain ends, are included. This transfer is driven by the external electric field, and opposed by the chemical hardness of each ${\mathrm{H}}_{2}$ unit. Unlike the situation in a bulk solid, this charge transfer is not suppressed (or even much affected) by electrostatic interactions among the transferred charges for $n\ensuremath{\leqslant}7$. Self-interaction-free approaches increase the chemical hardness of an ${\mathrm{H}}_{2}$ unit in comparison with semilocal density functionals, and so reduce the charge transfer. The physical picture behind the model is validated and its limitations are revealed by an analysis of the charge density from self-consistent electronic structure calculations. An appendix presents an accurate method to extract the hyperpolarizability from self-consistent calculations, and its results.
