Publications

Energetics of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>MnO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>polymorphs in density functional theory

We report the energetics and properties of the β, α, R, γ, λ, and δ polymorphs of MnO&lt;sub&gt;2&lt;/sub&gt; within density functional theory, comparing the performance of the recently introduced SCAN functional with that of conventional exchange-correlation functionals and experiment. We find that SCAN uniquely yields accurate formation energies and properties across all MnO&lt;sub&gt;2&lt;/sub&gt; polymorphs. We explain the superior performance of SCAN based on its satisfaction of all known constraints appropriate to a semilocal exchange-correlation functional and its accurate representation of all types of orbital overlap.

Reduced density-gradient analysis of atoms and molecules

A new pseudopotential for simple metals

Emergence of competing electronic states from non-integer nuclear charges

Abstract Understanding many-electron phenomena with competing near-degenerate electronic states is of fundamental importance to chemistry and condensed matter physics. One of the most significant challenges for exploring such many-electron phenomena is the necessity for large system sizes in order to realize competing states, far beyond those practical for first-principles methods. Here, we show how allowing non-integer nuclear charges expands the space of computationally tractable electron systems that host competing electronic states. The emergence of competing electronic states from non-integer nuclear charges is exemplified in the simple 2-electron H 2 molecule and used to examine the microscopic structure of doped quasi-1D cuprate chains, showing how non-integer nuclear charges can open a window for first-principles calculations of difficult many-electron phenomena.

Generalized gradient approximation to the exchange hole

Density functionals combined with van der Waals corrections for graphene adsorbed on layered materials

Standard density functionals like Perdew-Burke-Ernzerhof (PBE) or Strongly Constrained and Appropriately Normed (SCAN) need a correction to account for long-range van der Waals (vdW) interaction. The damped Zaremba-Kohn model (dZK) [J. Tao, H. Tang, A. Patra, P. Bhattarai, and J. P. Perdew, Phys. Rev. B 97, 165403 (2018)] starts from a formula for the vdW interaction of a distant atom with a solid surface, both with known dielectric properties, damps this formula at short range, and then treats an adsorbed molecule or atomic layer as a collection of renormalized atoms. We extend this model to graphene adsorbed on semiconducting layered materials [bulk graphite and hBN, and multilayer transition metal dichalcogenides (TMD)] by including the ${C}_{4}$ asymptotic term and multiple electrostatic image effects due to the two surfaces of the substrate slabs in the vdW calculations. The resulting SCAN-vdW-dZK and PBE-vdW-dZK give approximately the same results for the systems considered here, in agreement with available reference values. The predicted binding energies are roughly 25% lower than those from SCAN+rVV10 (revised Vydrov and Van Voorhis 2010), and $\ensuremath{\sim}15%$ lower than those from PBE+rVV10. Since SCAN+rVV10 usually overbinds, the predicted binding energies by SCAN-vdW-dZK and PBE-vdW-dZK are expected to be closer to the true values. The predicted equilibrium binding distances from SCAN-vdW-dZK and PBE-vdW-dZK are slightly larger ($\ensuremath{\sim}0.1\phantom{\rule{0.16em}{0ex}}\AA{}$) than those from SCAN+rVV10, and close to those from SCAN. The binding energy depends upon the number of substrate layers more strongly in vdW-dZK than in rVV10. The ${C}_{4}$-term contributions can be 40% of the total vdW interactions, and the ${C}_{5}$ term contributes about 10%. The effects of images and the back surfaces of slabs can contribute about 4--10%. The vdW interaction energy power laws from the vdW-dZK model for graphene adsorbed on multilayer $\mathrm{Mo}{\mathrm{S}}_{2}$ show slowly varying decay to the pairwise exponent \ensuremath{-}4 with increasing separation $D$, very similar to those obtained from the random-phase approximation and renormalization group approaches by Ambrosetti et al. Both the PBE-vdW-dZK and SCAN-vdW-dZK give a greater increase in interlayer binding energy when the TMD substrate changes from monolayer to four-layer for the graphene/TMD adsorption systems. This is consistent with the relevant electron-energy-loss spectroscopy experimental results showing increased dielectric response from the substrate with increasing substrate layer number.

Dominant density parameters and local pseudopotentials for simple metals

The properties of the simple metals are controlled largely by three density parameters: the equilibrium average valence electron density 3/4\ensuremath{\pi}${\mathit{r}}_{\mathit{s}}^{3}$, the valence z, and the density on the surface of the Wigner-Seitz cell, represented here by the equilibrium number ${\mathit{N}}_{\mathrm{int}}$ of valence electrons in the interstitial region. To demonstrate this fact, and as a refinement of the ``stabilized jellium'' or ``structureless pseudopotential'' model, we propose a structured local electron-ion pseudopotential w(r) which depends upon either ${\mathit{r}}_{\mathit{s}}$ and z (``universal'' choice for ${\mathit{N}}_{\mathrm{i}\mathrm{n}\mathrm{t})}$, or ${\mathit{r}}_{\mathit{s}}$, z, and ${\mathit{N}}_{\mathrm{int}}$ for each metal (``individual'' potential). Calculated binding energies, bulk moduli, and pressure derivatives of bulk moduli, evaluated in second-order perturbation theory, are in good agreement with experiment for 16 simple metals, and the bulk moduli are somewhat better than those calculated from first-principles nonlocal norm-conserving pseudopotentials. Structural energy differences agree with those from a nonlocal pseudopotential calculation for Na, Mg, and Al, but not for Ca and Sr. Our local pseudopotential w(r) is analytic for all r, and displays an exponential decay of the core repulsion as r\ensuremath{\rightarrow}\ensuremath{\infty}. The decay length agrees with that of the highest atomic core orbital of s or p symmetry, corroborating the physical picture behind this ``evanescent core'' form. The Fourier transform or form factor w(Q) is also analytic, and decays rapidly as Q\ensuremath{\rightarrow}\ensuremath{\infty}; its first and only zero ${\mathit{Q}}_{0}$ is close to conventional or empirical values. In comparison with nonlocal pseudopotentials, local ones have the advantages of computational simplicity, physical transparency, and suitability for tests of density functional approximations against more-exact many-body methods.

Energetics of MnO[subscript 2] polymorphs in density functional theory

We report the energetics and properties of the β, α, R, γ, λ, and δ polymorphs of MnO[subscript 2] within density functional theory, comparing the performance of the recently introduced SCAN functional with that of conventional exchange-correlation functionals and experiment. We find that SCAN uniquely yields accurate formation energies and properties across all MnO[subscript 2] polymorphs. We explain the superior performance of SCAN based on its satisfaction of all known constraints appropriate to a semilocal exchange-correlation functional and its accurate representation of all types of orbital overlap.

Kohn-Sham exact-exchange for atoms and clusters

Can Density Functional Theory Describe Strongly Correlated Electronic Systems?

Accurate first-principles description of the antiferromagnetic state of La 2 CuO 4

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

Density functional theory of autodetaching states

The authors consider the application of ab initio single-configuration orbital theories to predict the existence and energies of autodetaching states or resonances in atoms and molecules. Numerical results are given for the (ns)2 resonances in H- and He. In some cases (e.g. He) the conventional orbital theories (either Hartree-Fock or the local spin density approximation for exchange and correlation) give useful results, but the important autodetaching states of negative ions are usefully described only by the new self-interaction-corrected (SIC) version of the local spin density approximation. The authors also discuss how a (1s)2 resonance appears and then disappears in a two-electron ion as the nuclear charge Z is reduced below 1. The SIC calculation for this system displays an analogue to the 'bound state in the continuum' which Stillinger (1966) found for Z approximately=0.9.

Understanding the Boron–Nitrogen Interaction and Its Possible Implications in Drug Design B

2-Aminoethoxydiphenylborate (2-APB) is a broad-spectrum modulator of various membrane proteins. Specifically, it exhibits concentration dependent modulation of calcium signaling through store-operated calcium (SOC) channels: low micromolar concentration of 2-APB stimulates SOC entry while a higher concentration induces complete inhibition. Ab initio quantum chemical calculations show that the relative stability of the two major isomers of 2-APB (cyclic and extended) is about 8 kcal/mol. The dual functionality of 2-APB for SOC channels is thus likely associated with its ability to switch among isomeric forms, suited to different binding sites in the SOC channels with distinct binding affinities. Importantly, the moderate relative stability of different isomers results from a delicate balance between the intramolecular boron–nitrogen coordinate bond with strength about −45 kcal/mol and ring strain engendered by cyclic oligomerization. The synergistic effect of these two factors likely makes 2-APB an ideal dual effect drug.

Self-interaction correction to density-functional approximations for many-electron systems

The exact density functional for the ground-state energy is strictly self-interaction-free (i.e., orbitals demonstrably do not self-interact), but many approximations to it, including the local-spin-density (LSD) approximation for exchange and correlation, are not. We present two related methods for the self-interaction correction (SIC) of any density functional for the energy; correction of the self-consistent one-electron potenial follows naturally from the variational principle. Both methods are sanctioned by the Hohenberg-Kohn theorem. Although the first method introduces an orbital-dependent single-particle potential, the second involves a local potential as in the Kohn-Sham scheme. We apply the first method to LSD and show that it properly conserves the number content of the exchange-correlation hole, while substantially improving the description of its shape. We apply this method to a number of physical problems, where the uncorrected LSD approach produces systematic errors. We find systematic improvements, qualitative as well as quantitative, from this simple correction. Benefits of SIC in atomic calculations include (i) improved values for the total energy and for the separate exchange and correlation pieces of it, (ii) accurate binding energies of negative ions, which are wrongly unstable in LSD, (iii) more accurate electron densities, (iv) orbital eigenvalues that closely approximate physical removal energies, including relaxation, and (v) correct longrange behavior of the potential and density. It appears that SIC can also remedy the LSD underestimate of the band gaps in insulators (as shown by numerical calculations for the rare-gas solids and CuCl), and the LSD overestimate of the cohesive energies of transition metals. The LSD spin splitting in atomic Ni and $s\ensuremath{-}d$ interconfigurational energies of transition elements are almost unchanged by SIC. We also discuss the admissibility of fractional occupation numbers, and present a parametrization of the electron-gas correlation energy at any density, based on the recent results of Ceperley and Alder.

Spherical vs. Non-Spherical and Symmetry-Preserving vs. Symmetry-Breaking Densities of Open-Shell Atoms in Density Functional Theory

The atomization energies of molecules from first-principles density functional approximations improve from the local spin-density approximation (LSDA) to the Perdew-Burke-Ernzerhof (PBE)) generalized gradient approximation (GGA) to the strongly constrained and appropriately normed (SCAN) meta-GGA, and their sensitivities to non-spherical components of the density increase in the same order. Thus, these functional advances increase density sensitivity and imitate the exact constrained search over correlated wavefunctions better than that over ensembles. The diatomic molecules studied here, singlet C2 and F2 plus triplet B2 and O2, have cylindrically symmetric densities. Because the densities of the corresponding atoms are non-spherical, the approximate Kohn-Sham potentials for the atoms have a lower symmetry than that of the external (nuclear) potential, so that the non-interacting wavefunctions are not eigenstates of the square of total orbital angular momentum, breaking a symmetry that yields a feature of the exact ground-state density. That spatial symmetry can be preserved by a non-self-consistent approach in which a self-consistent equilibrium-ensemble calculation is followed by integer re-occupation of the Kohn-Sham orbitals, as the first of several steps. The symmetry-preserving approach is different from symmetry restoration based upon projection. First-step space- (and space-spin-) symmetry preservation in atoms is shown to have a small effect on the atomization energies of molecules, quantifying earlier observations by Fertig and Kohn. Thus, the standard Kohn-Sham way of calculating atomization energies, with self-consistent symmetry breaking to minimize the energy, is justified, at least for the common cases where the molecules cannot break symmetry.

Copper-Intercalated Birnessite as a Water Oxidation Catalyst

We report a synthetic method to increase the catalytic activity of birnessite toward water oxidation by intercalating copper in the interlayer region of the layered manganese oxide. Intercalation of copper, verified by XRD, XPS, ICP, and Raman spectroscopy, was accomplished by exposing a suspension of birnessite to a Cu(+)-bearing precursor molecule that underwent disproportionation in solution to yield Cu(0) and Cu(2+). Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to ∼700 mV (at 10 mA/cm(2)) and 240 mV/decade, respectively, for birnessite without copper. Impedance spectroscopy results suggested that the charge transfer resistivity of the Cu-modified sample was significantly lower than Cu-free birnessite, suggesting that Cu in the interlayer increased the conductivity of birnessite leading to an enhancement of water oxidation kinetics. Density functional theory calculations show that the intercalation of Cu(0) into a layered MnO2 model structure led to a change of the electronic properties of the material from a semiconductor to a metallic-like structure. This conclusion from computation is in general agreement with the aforementioned impedance spectroscopy results. X-ray photoelectron spectroscopy (XPS) showed that Cu(0) coexisted with Cu(2+) in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu(2+) showed a reduction in water oxidation kinetics, further emphasizing the importance of Cu(0) for the increased activity of birnessite. The introduction of Cu(0) into the birnessite structure also increased the stability of the electrocatalyst. At a working current of 2 mA, the Cu-modified birnessite took ∼3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.

Importance of van der Waals correction for energetics in transition metal monoxides