Use of gradient-corrected functionals in total-energy calculations for solids

Uncertainty of DFT Calculated Mechanical and Structural Properties of Solids due to Incompatibility of Pseudopotentials and Exchange-Correlation Functionals

The demand for pseudopotentials constructed for a given exchange-correlation (XC) functional far exceeds the supply, necessitating the use of those commonly available. The number of XC functionals currently available is in the hundreds, if not thousands, and the majority of pseudopotentials have been generated for LDA and PBE. The objective of this study is to identify the error in the determination of the mechanical and structural properties (lattice constant, cohesive energy, surface energy, elastic constants, and bulk modulus) of crystals calculated by DFT with such inconsistency. Additionally, this study aims to estimate the performance of popular XC functionals (LDA, PBE, PBEsol, and SCAN) for these calculations in a consistent manner.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Density Functional Theoretical Calculations for a Co2/γ-Al2O3 Model Catalyst: Structures of the γ-Al2O3 Bulk and Surface and Attachment Sites for Co2+ Ions

First-principle density functional theory (DFT) calculations on the electronic state and structure of a [Co2+]2/gamma-Al2O3 model catalyst have been performed in relation to catalysis for unique NO-CO reactions on a Co2+ ensemble/gamma-Al2O3 catalyst. The DFT calculations reveal that a bulk structure of gamma-Al2O3 is energetically most favorable when aluminum vacancies are evenly dispersed at octahedral sites, and that the (110) plane is exposed as a top-most layer by its neutrality. Two Co2+ ions on the (110) surface are supported adjacently to each other in a tetrahedral symmetry. The calculations also demonstrate that the vacant d orbitals of the two Co2+ ions are directed toward each other, which brings about an adsorbate-adsorbate interaction between two molecules which adsorb on each of the Co2+ ions. This may be an origin of the unique aspect of Co2+ ensemble/gamma-Al2O3 catalysis.

Density-functional calculation of CeO2 surfaces and prediction of effects of oxygen partial pressure and temperature on stabilities

We have used density-functional theory to investigate (111), (110), (210), (211), (100), and (310) surfaces of ceria (CeO2). Compared with previous interatomic-potential-based studies, our calculations reported a slightly different relative stability ordering and significantly lower surface energies for the stoichiometric surfaces. Using a defect model, the surface stabilities were evaluated as functions of oxygen partial pressure and temperature. Our investigations were restricted to ideal surface terminations, without considering defect formation on those surfaces. We found that at 300 K, the stoichiometric (111) has the lowest free energy for a wide range of oxygen partial pressures up to 1 atm, and only at ultrahigh vacuum does the Ce-terminated (111) becomes the most stable one. The transition point for the Ce-terminated (111) surfaces moves to higher oxygen partial pressures when temperature increases. To improve the prediction of electron density of states, we used the local-density approximation plus U(J) correction method to correct the on-site Coulomb correlation and exchange interaction due to the strongly localized Ce-4f electrons. The optimal parameter combination of U = 7 eV and J = 0.7 eV was found to improve the O 2p-Ce 4f gap without much degradation of ground-state bulk properties or the O 2p-Ce 5d gap. The bulk and surface electronic structures were then analyzed based on the improved density of states.

Finding transition states for crystalline solid-solid phase transformations

We present a method to identify transition states and minimum energy paths for martensitic solid-solid phase transformations, thereby allowing quantification of the activation energies of such transformations. Our approach is a generalization of a previous method for identifying transition states for chemical reactions, namely the climbing image-nudged elastic band algorithm, where here the global deformation of the crystalline lattice (volume and shape fluctuations) becomes the reaction coordinate instead of atomic motion. We also introduce an analogue to the Born-Oppenheimer approximation that allows a decoupling of nuclear motion and lattice deformation, where the nuclear positions along the path are determined variationally according to current deformation state. We then apply this technique to characterize the energetics of elemental lithium phase transformations as a function of applied pressure, where we see a validation of the Born-Oppenheimer-like approximation, small energy barriers (expected for martensitic transformations), and a pronounced pressure dependence of various properties characterizing the phase transitions.

Effects of Water on the Structure and Bonding of Resorcinol in the Interlayer of Montmorillonite Nanocomposite: A Periodic First Principle Study

Resorcinol forms a novel nanocomposite in the interlayer of montmorillonite. This resorcinol oligomer is stable inside the clay matrixes even above the boiling point of the monomer. A periodic ab initio calculation was performed with hydrated and nonhydrated montmorillonite before and after intercalation of resorcinol. For the most feasible dimer and tetramer shaped oligomer of resorcinol, the intramolecular and intermolecular hydrogen bonding feasibility has been tested using the DFT-BLYP approach and the DNP basis set in the gas phase and in the presence of aqueous solvent. After locating the active site through Fukui functions within the helm of the hard-soft acid-base principle, the relative nucleophilicity of the active cation sites in their hydrated state has been calculated. A novel quantitative scale in terms of the relative nucleophilicity and electrophilicity of the interacting resorcinol oligomers before and after solvation is proposed. Besides that, a comparison with a hydration situation and also the strength of the hydrogen bridges have been evaluated using mainly the dimer and cyclic tetramer type oligomers of resorcinol. Using periodic ab initio calculations, the formation mechanism was traced by the following two ways: (1) resorcinol molecules combine without any interaction with water or (2) resorcinol oligomerizes through water. Both the mechanism is compared and the effect of water on the process is elucidated. The results show that resorcinol molecules combine after hydration only and hence they are stable at higher temperature. The fittings of the oligomers were also tested as well by periodic calculation to compare the stability of the oligomers inside the newly formed clay nanocomposite.

Effect of exchangeable cation on the swelling property of 2:1 dioctahedral smectite—A periodic first principle study

We used both localized and periodic calculations on a series of monovalent (Li+, Na+, K+, Rb+, Cs+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations to monitor their effect on the swelling of clays. The activity order obtained for the exchangeable cations among all the monovalent and divalent series studied: Ca2+ > Sr2+ > Mg2+ > Rb+ > Ba2+ > Na+ > Li+ > Cs+ > K+. We have shown that, in case of dioctahedral smectite, the hydroxyl groups play a major role in their interaction with water and other polar molecules in the presence of an interlayer cation. We studied both type of clays, with a different surface structure and with/without water using a periodic calculation. Interlayer cations and charged 2:1 clay surfaces interact strongly with polar solvents; when it is in an aqueous medium, clay expands and the phenomenon is known as crystalline swelling. The extent of swelling is controlled by a balance between relatively strong swelling forces and electrostatic forces of attraction between the negatively charged phyllosilicate layer and the positively charged interlayer cation. We have calculated the solvation energy at the first hydration shell of an exchangeable cation, but the results do not correspond directly to the experimental d-spacing values. A novel quantitative scale is proposed with the numbers generated by the relative nucleophilicity of the active cation sites in their hydrated state through Fukui functions within the helm of the hard soft acid base principle. The solvation effect thus measured show a perfect match with experiment, which proposes that the reactivity index calculation with a first hydration shell could rationalize the swelling mechanism for exchangeable cations. The conformers after electron donation or acceptance propose the swelling mechanism for monovalent and divalent cations.

AB INITIO DYNAMICS OF SURFACE CHEMISTRY

▪ Abstract  We review the young field of ab initio molecular dynamics applied to molecule-surface reactions. The techniques of ab initio molecular dynamics include methods that use an analytic potential energy function fit to ab initio data and those that are fully ab initio. In this review, we focus on the insights provided by ab initio–based molecular dynamics that are currently unavailable from experimental studies and discuss current techniques and limitations. As an example of how different aspects of a problem can be tackled with state-of-the-art theoretical tools, we consider the well-studied case of H2 desorption and adsorption from the Si(100)-2 × 1 surface.