First-principles Energy Research Articles

Rotational and steric effects in water dissociative chemisorption on Ni(111).

Powerful laser techniques have recently enabled quantum-state resolved molecular beam experiments for investigating gas-surface reactions, which have unveiled intriguing vibrational, rotational, and also steric effects. For reactions involving polyatomic molecules, e.g., the dissociative chemisorption of methane and water, the rotational and related steric effects are far less understood despite a large body of theoretical work having been able to reproduce the observed vibrational mode specificity and related bond selectivity semi-quantitatively or even within chemical accuracy. Herein, we report a high dimensional quantum dynamics study of water dissociation on Ni(111) on a first-principles potential energy surface, focusing on the reactivities of D2O in various rotational quantum states with different spatial orientations. Through an accurate quantum mechanical description of this asymmetric top, remarkable dependence of the reactivity on the orientation is observed. This dependence is site specific and rotational state specific. These single site rotational and steric effects are partially justified by a sudden model on the basis of the overlap between the rotational wavefunctions and the angular potential near the transition state, but rotational steering also plays a significant role which complicates the dynamics. Although site averaging weakens the influence of initial rotational excitations and leads to minor effects to the reactivity, steric effects are predicted to be observable if the water molecule is selectively excited and aligned by a linearly polarized laser.

Semi-physical Identification and State Estimation of Energy Intake for Interventions to Manage Gestational Weight Gain.

Excessive gestational weight gain (i.e., weight gain during pregnancy) is a significant public health concern, and has been the recent focus of novel, control systems-based interventions. This paper develops a control-oriented dynamical systems model based on a first-principles energy balance model from the literature, which is evaluated against participant data from a study targeted to obese and overweight pregnant women. The results indicate significant under-reporting of energy intake among the participant population. A series of approaches based on system identification and state estimation are developed in the paper to better understand and characterize the extent of under-reporting; these range from back-calculating energy intake from a closed-form of the energy balance model, to a constrained semi-physical identification approach that estimates the extent of systematic under-reporting in the presence of noise and possibly missing data. Additionally, we describe an adaptive algorithm based on Kalman filtering to estimate energy intake in real-time. The approaches are illustrated with data from both simulated and actual intervention participants.

Accurate first-principles structures and energies of diversely bonded systems from an efficient density functional.

One atom or molecule binds to another through various types of bond, the strengths of which range from several meV to several eV. Although some computational methods can provide accurate descriptions of all bond types, those methods are not efficient enough for many studies (for example, large systems, ab initio molecular dynamics and high-throughput searches for functional materials). Here, we show that the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) within the density functional theory framework predicts accurate geometries and energies of diversely bonded molecules and materials (including covalent, metallic, ionic, hydrogen and van der Waals bonds). This represents a significant improvement at comparable efficiency over its predecessors, the GGAs that currently dominate materials computation. Often, SCAN matches or improves on the accuracy of a computationally expensive hybrid functional, at almost-GGA cost. SCAN is therefore expected to have a broad impact on chemistry and materials science.

Complex Precipitation Sequences of Al-Cu-Li-(Mg) Alloys Characterized in Relation to Thermal Ageing Processes

The Al-Cu-Li-(Mg) alloy is a high-performance lightweight material strengthened by complex coexisting precipitates that form in the alloy upon thermal ageing. Using high-resolution (scanning) transmission electron microscopy in association with first-principles energy calculations, we systematically studied the complex coexisting precipitates in the alloys and correlated their precipitation sequences with thermal ageing processes applied. The principal results are the following: (1) eight types of precipitates can be observed in the alloy; (2) of these precipitates, the T 1-phase is most stable. The S-phase precipitates with segregated Li atoms at their interfacial edges are unexpectedly more stable than the σ-phase; (3) the T 1-phase has a characteristic precursor that plays the key role in its nucleation and growth.

Anisotropic electronic conduction in stacked two-dimensional titanium carbide.

Stacked two-dimensional titanium carbide is an emerging conductive material for electrochemical energy storage which requires an understanding of the intrinsic electronic conduction. Here we report the electronic conduction properties of stacked Ti3C2T2 (T = OH, O, F) with two distinct stacking sequences (Bernal and simple hexagonal). On the basis of first-principles calculations and energy band theory analysis, both stacking sequences give rise to metallic conduction with Ti 3d electrons contributing most to the conduction. The conduction is also significantly anisotropic due to the fact that the effective masses of carriers including electrons and holes are remarkably direction-dependent. Such an anisotropic electronic conduction is evidenced by the I−V curves of an individual Ti3C2T2 particulate, which demonstrates that the in-plane electrical conduction is at least one order of magnitude higher than that vertical to the basal plane.

First-principles energy band calculation of Ruddlesden–Popper compound Sr3Sn2O7 using modified Becke–Johnson exchange potential

The electronic structure of Sr3Sn2O7 is evaluated by the scalar-relativistic full potential linearized augmented plane wave (FLAPW+lo) method using the modified Becke–Johnson potential (Tran–Blaha potential) combined with the local density approximation correlation (MBJ–LDA). The fundamental gap between the valence band (VB) and conduction band (CB) is estimated to be 3.96eV, which is close to the experimental value. Sn 5s states and Sr 4d states are predominant in the lower and upper CB, respectively. On the other hand, the lower VB is mainly composed of Sn 5s, 5p, and O 2p states, while the upper VB mainly consists of O 2p states. These features of the DOS are well reflected by the optical transition between the upper VB and lower CB, as seen in the energy dependence of the dielectric function. Furthermore, the absorption coefficient estimated from the MBJ–LDA is similar to the experimental result.

The possibilities to lower the stacking fault energies of aluminum materials investigated by first-principles energy calculations

The stacking fault energies (SFEs) of various aluminum solid solutions were investigated against temperature and solute concentration using first-principles energy calculations. Three types of impurities, including intrinsic defects, substitutional solute atoms and interstitial solutes were considered to study their effects on the SFEs in corresponding aluminum solid solutions. It is shown that the intrinsic defects hardly change the SFEs of aluminum, whereas substitutional solutes such as Ge, Y, Sc, Sr and interstitial solutes such as C, N, H, can drastically lower the SFEs of aluminum at low temperatures. Our study suggests that at circumstances the possibilities to introduce twins in aluminum materials can be increased significantly, since their SFEs can be lowered to the values smaller than 45mJ/m2. The physical mechanism of lowering the SFEs in those aluminum solid solutions were interpreted by their electronic structures around the SFs and the solutes or impurities.

Electronic structures of ZnX (X = O and S) nanosheets from first-principles energy loss near edge structure studies

This paper tries to study the core energy loss spectra of zinc based nanosheets (ZnO and ZnS) in density functional theory using the FPLAPW method. We have calculated the energy loss near edge structure (ELNES) spectra of zinc K- and L2,3-edges, and oxygen and sulfur K-edges in ZnO and ZnS nanosheet at magic angle conditions and compare to those of ZnO and ZnS wurtzite bulk structures. As a result of the smaller bond lengths in the nanosheet structures, all the ELNES spectra in nanosheets, including Zn K- and L2,3-edges, and O and S K-edges show a shift to the higher energies. The calculations reveal that in comparison to Zn edges in ZnO structures, all the ELNES spectra of ZnS structures including the bulk and sheet show a shift to lower energy region. This is a result of larger bond lengths in ZnS structures, and that it can be used to fingerprint each structure. The comparison of ELNES spectra and unoccupied symmetry-projected density of states (local DOS) confirms that Zn K-edges of both ZnO and ZnS nanosheets have main features including the electron transition to π* and σ* states which this is a sign of the strong sp2 hybridization and it is absent in Zn K-edge of ZnO and ZnS bulks. Moreover, in comparison to bulks, the energy differences between some main features in Zn based nanosheets decreases. The accordance of ELNES spectra and unoccupied local DOS shows that the main contributions in Zn L-edges ELNES of ZnO and ZnS nanosheets refer to transitions to mostly d-symmetry states and p-d hybridization. Orientation dependencies of the spectral features in nanosheet structures were also consideable. The overall dispersions of oxygen and sulfur K-edges in bulks and nanosheets are different. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in ZnO and ZnS bulks and due to the rather wide band gaps of Zn based nanosheets, it may be essential as well for all the ELNES spectra of nanosheet structures.

First-principles energy band calculation and one step synthesis of N-doped BiPO4

N-doped BiPO4 photocatalyst was synthesized by a simple one step hydrothermal method. The crystal structure, morphology, and photophysical properties of the samples were characterized by XRD, SEM, XPS, UV–Vis and Raman. The valence and conduction bands, density of states and the band gap energy of BiPO4 are calculated with first-principles. The results showed that both BiPO4 and N-doped BiPO4 have same monoclinic structure. The substitution of O2− by N3− ions in BiPO4 could not only cause the red-shift of the adsorption edge, but also change the morphology. The photocatalytic activity of BiPO4 was enhanced about 50% under UV light irradiation when the molar ratio of N/Bi is 0.2, this can be ascribed to the N3− introduced into BiPO4 crystal lattice restrain the recombination of photogenerated electron–hole pairs. However, overdoping would decrease the photocatalytic activity of BiPO4.

Nature of the Asymmetry in the Hydrogen-Bond Networks of Hexagonal Ice and Liquid Water

The interpretation of the X-ray spectra of water as evidence for its asymmetric structure has challenged the traditional nearly tetrahedral model and initiated an intense debate about the order and symmetry of the hydrogen-bond network in water. Here, we present new insights into the nature of local interactions in ice and liquid water obtained using a first-principle energy decomposition method. A comparative analysis shows that the majority of molecules in liquid water in our simulation exhibit hydrogen-bonding energy patterns similar to those in ice and retain the four-fold coordination with only moderately distorted tetrahedral configurations. Although this result indicates that the traditional description of liquid water is fundamentally correct, our study also demonstrates that for a significant fraction of molecules the hydrogen-bonding environments are highly asymmetric with extremely weak and distorted bonds.

Three-band tight-binding model for monolayers of group-VIB transition metal dichalcogenides

We present a three-band tight-binding (TB) model for describing the low-energy physics in monolayers of group-VIB transition metal dichalcogenides $MX_2$ ($M$=Mo, W; $X$=S, Se, Te). As the conduction and valence band edges are predominantly contributed by the $d_{z^{2}}$, $d_{xy}$, and $d_{x^{2}-y^{2}}$ orbitals of $M$ atoms, the TB model is constructed using these three orbitals based on the symmetries of the monolayers. Parameters of the TB model are fitted from the first-principles energy bands for all $MX_2$ monolayers. The TB model involving only the nearest-neighbor $M$-$M$ hoppings is sufficient to capture the band-edge properties in the $\pm K$ valleys, including the energy dispersions as well as the Berry curvatures. The TB model involving up to the third-nearest-neighbor $M$-$M$ hoppings can well reproduce the energy bands in the entire Brillouin zone. Spin-orbit coupling in valence bands is well accounted for by including the on-site spin-orbit interactions of $M$ atoms. The conduction band also exhibits a small valley-dependent spin splitting which has an overall sign difference between Mo$X_{2}$ and W$X_{2}$. We discuss the origins of these corrections to the three-band model. The three-band TB model developed here is efficient to account for low-energy physics in $MX_2$ monolayers, and its simplicity can be particularly useful in the study of many-body physics and physics of edge states.

Study of interaction between radioactive nuclides and graphite surface by the first-principles and statistic physics

The adsorption and desorption of four kinds of main radioactive productions (cesium, iodine, strontium and silver) on graphite surface in high temperature gas cooled reactors (HTGRs) have been studied. Using the first-principles density-functional theory, adsorptive geometry, energy and electron structure on the perfect and defective graphite surfaces have been calculated. It turns out that the adsorption of Cs, I and Sr atoms belongs to chemisorption while the adsorption of Ag is a pure physisorption. When introducing a vacancy in graphite surface, nuclide adatoms will be trapped by the vacancy and form chemical bonds with three nearest neighbor carbon atoms, leading to significant increase of the adsorption energy. In addition, a model of grand canonical ensemble is employed to deduce the adsorption rate as a function of temperature and partial pressure of nuclides produced. The transition temperate from adsorption to desorption of nuclides on graphite surface is defined as the inflexion point of the adsorption rate and its variation with nuclide density is obtained.

Liquid–liquid transition in supercooled water suggested by microsecond simulations

The putative liquid-liquid phase transition in supercooled water has been used to explain many anomalous behaviors of water. However, no direct experimental verification of such a phase transition has been accomplished, and theoretical studies from different simulations contradict each other. We investigated the putative liquid-liquid phase transition using the Water potential from Adaptive Force Matching for Ice and Liquid (WAIL). The simulation reveals a first-order phase transition in the supercooled regime with the critical point at ~207 K and 50 MPa. Normal water is high-density liquid (HDL). Low-density liquid (LDL) emerges at lower temperatures. The LDL phase has a density only slightly larger than that of the ice-Ih and shows more long-range order than HDL. However, the transformation from LDL to HDL is spontaneous across the first-order phase transition line, suggesting the LDL configuration is not poorly formed nanocrystalline ice. It has been demonstrated in the past that the WAIL potential provides reliable predictions of water properties such as melting temperature and temperature of maximum density. Compared with other simple water potentials, WAIL is not biased by fitting to experimental properties, and simulation with this potential reflects the prediction of a high-quality first-principle potential energy surface.

Solute effect on oxygen diffusion in α-titanium

We calculate first-principles interaction energies between substitutional solutes and oxygen interstitials in α-titanium and predict the effect of solutes on oxygen diffusion from those interactions. Interaction between 45 solutes across the periodic table and three oxygen interstitial sites are calculated with density-functional theory. The interaction energies show distinct trends across the periodic table corresponding to both atomic radii and the period. Changes in diffusion barrier due to solutes are modeled with the kinetically resolved activation barrier approximation. Solute effects at infinite dilution are numerically calculated and show both accelerated and reduced oxygen diffusivity.

CORE-HOLE EFFECT IN THE Ce L3 X-RAY ABSORPTION SPECTRA OF CeO2 AND CeFe2: NEW EXAMINATION BY USING RESONANT X-RAY EMISSION SPECTROSCOPY

We consider two different resonant X-ray emission spectra for Ce compounds: Ce 3d to 2p X-ray emission (denoted by 3d-RXES) and valence to 2p X-ray emission (v-RXES), both of which follow the Ce 2p to 5d resonant excitation. We propose that the comparison of the 3d- and v-RXES spectra is a new powerful method of directly detecting the core-hole effect in the final state of Ce L 3 X-ray absorption spectra (XAS). We applied this method to recent experimental RXES spectra for CeO 2 and CeFe 2, and showed unambiguously that the core-hole effect should be essential in the XAS of both materials. This result is confirmed by theoretical calculations, which reproduce well the experimental RXES and XAS spectra. We conclude that the ground state of CeO 2 is in the mixed state of 4f0 and [Formula: see text] configurations, where [Formula: see text] is a ligand hole, instead of a pure 4f0 configuration which was proposed recently by first-principles energy band calculations. Also, we conclude that the double peaks observed in L 3 XAS of CeFe 2 are caused by the 4f0 and 4f1 configurations, which are mixed in the ground state but separated in energy by the large core-hole potential in the final state of XAS.

The study of oxygen and sulfur adsorption on the InSb (110) surface, using first-principle energy calculations

基于第一性原理方法, 采用广义梯度近似的交换关联势, 对InSb材料(110)表面的硫吸附和氧吸附之后体系性质的差异进行了分析. 讨论了两种吸附下的键长、键角、能带结构和态密度的变化, 从理论上论证了硫吸附比氧吸附对InSb红外探测器表面态的钝化有优越性, 有利于工艺上在钝化时的选择.

The crystallographic and morphological evolution of the strengthening precipitates in Cu–Ni–Si alloys

High-resolution transmission electron microscopy and first-principles energy calculations reveal that, upon formation, the hardening precipitates in Cu–Ni–Si alloys are unchanged δ-Ni2Si nanocrystals. However, their crystallographic and morphological features evolve during the precipitation process. It is shown that, in terms of crystallographic orientation relationships, there are basically two types of δ-Ni2Si precipitates in the alloys, referred to as δ1-Ni2Si and δ2-Ni2Si respectively. In the early stages of aging (including peak aging), the precipitates are small and belong to the δ1 type, with the following orientation relationship with the Cu matrix: [010]δ||[110]Cu and (001)δ||(001)Cu. In the late stages, the precipitates are clearly larger and become the δ2 type, with the orientation relationship: [010]δ||[110]Cu and approximately (301)δ‖(11¯1)Cu. Governed by the minimization of its overall energy, a developing δ precipitate has to evolve from an almond-like δ1 particle with a low-index coherent habit plane to a French baguette bread-slice-shaped δ2 particle that has a high-index broad interface. This evolution is found to be in excellent agreement with predictions provided by the invariant line theory. Intermediate stages exist for a particle to accomplish such an evolution, leading to many different crystallographic and morphological appearances of these δ-Ni2Si particles being observed in the alloys.

Rate coefficient for the important interstellar radiative association between [formula omitted] and H2 from classical reaction dynamics

The CH3++H2→CH5++hν radiative associative reaction plays a major role in the synthesis of complex hydrocarbons in interstellar dust clouds and was studied using a part-classical, part-ab initio approach. A first-principles potential energy surface (PES) at CCSD/6-311++G(2df, 2pd) was constructed from a set of 828 data points in configurational space using modified Shepard interpolation, and was used in conjuction with long-range perturbation theory at large intermolecular distances. Data points are chosen so as to optimize PES construction according to the GROW software algorithm. Classical reaction dynamics was employed to infer the reaction rate coefficient.

Double-atomic-wall-based dynamic precipitates of the early-stage S-phase in AlCuMg alloys

Developments of high-strength aluminum alloys have always faced a difficult problem: owing to their small size, the early-stage strengthening precipitates are difficult to characterize in terms of composition, structure and evolution. Even for the widely used AlCuMg alloys, in which the phenomenon of precipitation hardening in metals was first discovered by Wilm more than a century ago, the essential questions remain: how many different precursors exist for the most effective strengthening precipitates (referred to S-phase), and how do they transform to the S-phase? Here we employ atomic-resolution electron microscopy imaging and first-principles energy calculations to address these problems. Our study demonstrates that the early-stage S-phase precipitates are highly dynamic in both composition and structure. Having their own genetic double Cu–Mg atomic walls to guide their evolution, these dynamic precipitates initiate, mature and grow with thermal aging following three evolution paths, leading to the S-phase precipitates formed, without exception, with even numbers of Cu–Mg atomic layers.

Computational modeling of broadband acoustic fields in enclosures with specular reflection boundaries using a first-principles energy method

Steady-state sound fields in enclosures with specular reflection boundaries are modeled with a first-principle energy-intensity boundary element method using uncorrelated broadband directional sources. The specular reflection field is represented by a limited set of spherical harmonics, orthogonal on the half-space. For each boundary element, the amplitudes of these harmonics are determined from the incident field from all other elements and sources, and are subject to an energy conservation integral constraint using a Lagrange multiplier method. The computational problem is solved using an iterative relaxation method starting from the 3-D diffuse reflection solution. At each iteration, directivity harmonics are estimated by post-processing and the influence matrix is refined accordingly. For internal sources, simple first reflection images improve accuracy with virtually no penalty on computation time. Convergence occurs in relatively few relaxation steps. Extrapolating to an infinite number of boundary elements and iterations gives very accurate results. Results are compared to exact benchmark solutions obtained from a frequency-by-frequency modal analysis, and to a broadband image method. The method of absorption scaling is verified for 3-D cases, and showing that the spatial variation in rooms is largely determined by source position and the relative distribution of absorption, but not the overall absorption level.