Effective Partial Charge Research Articles

Effect of Li2O on the structure and properties of low‐boron aluminosilicate fiber glasses from molecular dynamics simulations and quantitative structure–property relationship analysis

AbstractMixed alkaline earth (MgO, CaO) aluminosilicate glass fibers (MCAS) with and without boron are commonly used as reinforcements in plastic composites, and the fundamental understanding of the thermal and mechanical properties is essential to the design of new glass compositions to satisfy the growing demands in applications from renewable energy to lightweight structural components. In this work, a series of Li2O containing low‐boron MCAS fiber glasses have been studied by using molecular dynamic (MD) simulations with recently developed effective partial charge composition dependent boron potentials. Structural characteristics, such as pair distribution function, bond angle distribution, and neutron/X‐ray diffraction structure factors, were calculated, as well as properties such as elastic moduli and vibrational density of states. The addition of Li2O was found to improve the elastic moduli of the fiber glasses in excellent agreement with experimental results we reported earlier. The simulation results showed that the weakened network connectivity and decrease of tri‐/bridging oxygen have positively affected the lowering of liquid temperature, owing to the transformation to more boron Q2 and silicon/aluminon Q3. It is found that higher oxygen packing density, coordinated aluminum/boron species such as [AlO5] and [BO4] units, larger‐membered oxide rings, and intensified connections of [AlOx] and [SiO4] are the main reasons that lead to improved mechanical properties. MD‐based quantitative structure–property relationship analyses were performed and showed excellent correlations to measured properties, indicating that it is a promising approach to understand glass properties and design new glass compositions for functional applications.

Charge-Optimized Electrostatic Interaction Atom-Centered Neural Network Algorithm.

Machine-learning algorithms have been proposed to capture electrostatic interactions by using effective partial charges. These algorithms often rely on a pretrained model for partial charge prediction using density functional theory-calculated partial charges as references, which introduces complexity to the force field model. The accuracy of the trained model also depends on the reliability of charge partition methods, which can be dependent on the specific system and methodology employed. In this study, we propose an atom-centered neural network (ANN) algorithm that eliminates the need for reference charges. Our algorithm requires only a single NN model for each element to obtain both atomic energy and charges. These atomic charges are then employed to compute electrostatic energies using the Ewald summation algorithm. Subsequently, the force field model is trained on total energy and forces, with the inclusion of electrostatic energy. To evaluate the performance of our algorithm, we conducted tests on three benchmark systems, including a Ge slab with an O adatom system, a TiO2 crystalline system, and a Pd-O nanoparticle system. Our results demonstrate reasonably accurate predictions of partial charges and electrostatic interactions. This algorithm provides a self-consistent charge prediction strategy and possibilities for robust and reliable modeling of electrostatic interactions in machine-learning potentials.

Atomistic Understanding of Ion Exchange Strengthening of Boroaluminosilicate Glasses: Insights from Molecular Dynamics Simulations and QSPR Analysis.

Ion exchange (IOX) is an effective and widely used method to enhance mechanical properties of various glass products ranging from the touch screen of consumer electronics to window shields of airplanes and spacecrafts. IOX or chemical strengthening is achieved through the creation of a compressive surface layer on the glass product. Although widely studied experimentally, the fundamental understanding of the IOX strengthening process is still limited. In this work, we have applied large-scale atomistic simulations to understand IOX-induced mechanical property changes and their relation to the glass composition and structural characteristics. Two series of borosilicate glasses are studied to elucidate the composition effect, with boron oxide for silica and alumina for silica substitutions, respectively, on the mechanical properties of different levels of K+ to Na+ ion exchanges by using molecular dynamics (MD) simulations with a set of recently developed effective partial charge potentials. The linear network dilation coefficient (LNDC), a common measure of IOX behaviors, was calculated for each of the glass compositions. Quantitative structural property relationship (QSPR) analysis based on the MD-generated structural features was used to establish the structure-property correlations of mechanical and other properties. The results show strong composition dependence of the LNDC, hence the suitability of IOX strengthening. This behavior is discussed based on glass structure features of the glasses. It was found that glass compositions with a higher amount of mixed glass formers, higher network connectivity, and less complex components tend to show higher calculated LNDC and higher surface compressive stress. MD simulations, in combination with QSPR analysis, can thus provide atomistic insights into how the glass composition and structural characteristics affect IOX behaviors.

Structures of Vanadium-Containing Silicate and Borosilicate Glasses: Vanadium Potential Development and MD Simulations

Vanadium-containing multi-component oxide glasses have found a variety of applications ranging from energy storage, sensing, to nuclear waste disposal. However, their complex structures, partly caused by the co-existence of multi-oxidation states of vanadium ions, lead to the challenges in experiments to understand the structural features and structural origin of the property change of these glasses. In this work, we aimed to develop compatible vanadium-related parameters for a widely used effective partial charge potential set to enable the simulations of the vanadium-containing multi-component oxide glasses (Deng and Du, J. Am. Ceram. Soc. 102(2019)2482). Various vanadium containing oxide crystal structures and model glass compositions (with vanadium in different oxidation states) have been used as targets to fit and refine the newly developed parameters. Both the experimental crystal structures and mechanical properties were used in the fitting process. Glass structures were characterized by performing cation coordination number, pair distribution function, and bond angle distribution analysis. The obtained parameters have been shown to be able to simulate the vanadium-containing aluminosilicate, phosphate, and borate glasses and describe the local environments of vanadium ions with various oxidation states with reasonable accuracy in comparison with available experimental data. This development thus enables atomistic simulations of vanadium containing oxide glasses to understand the structural role of vanadium ions in these glasses and how the struacture features affect various glass properties hence assist the design of vanadium containing glasses for various technological applications.

In situ pair distribution function analysis of crystallizing Fe-silicate melts

The structures of a series of Na2O–FeO–Fe2O3–SiO2 melts with Si/Fe = 1, 2, or 3 have been characterized via synchrotron X-ray total scattering using aerodynamic levitation, a containerless technique, paired with laser heating. The melt structure has been simulated using empirical potential structure refinement (EPSR) based on X-ray scattering data and molecular dynamics (MD) simulations with effective partial charge potentials. Pair distribution functions and coordination numbers of cations in the melt were obtained from the data refinement and from the MD simulations. Iron redox and accompanying density were modeled for each composition assuming either (1) oxidized (all Fe3+) or (2) reduced (some Fe2+) to the level predicted for the Ar levitator environment from a literature model. The actual redox of the melts appears to be intermediate, with some Fe2+ but not as much as assumed from the literature model of redox. Comparison of EPSR and MD models at the same redox conditions indicates generally good agreement, and precise values of coordination numbers depend sensitively on the assumptions for cutoff lengths. Stepwise cooling of each melt in the series resulted in formation of magnetite on the free surface of the sample, but no silica-containing crystalline phases were observed. This study provides a comprehensive assessment of coupled factors (composition, redox, density) needed to assess high-temperature in situ structural measurements of simplified iron silicates.

Effects of Surface Orientation and Termination Plane on Glass‐to‐Crystal Transformation of Lithium Disilicate by Molecular Dynamics Simulations

Glass‐to‐crystal transformation of lithium disilicate is studied using molecular dynamics simulations using an effective partial charge potential. The structural evolution of the interface between glassy and crystalline lithium disilicate is analyzed to simulate crystallization of glass on pre‐existing crystal seeds. Besides previously used atomic number density, the distribution of Qn species (Si tetrahedra with n bridging oxygen) is shown to be an effective parameter for following this transformation quantitatively. The early stages of crystal growth are significantly affected by the orientation and termination of the surface of adjacent crystal, as indicated by calculated atomic density, partial ordering, atomic segregation, and an increase in Q3 concentration. In particular, under‐coordinated Si within the outer crystal layer is found to be most effective in transforming the amorphous structure toward crystallinity. The increase in Q3 in the glass close to interface region most clearly shows the initial stage of lithium disilicate crystal growth.

Structural Origins of RF3/NaRF4 Nanocrystal Precipitation from Phase-Separated SiO2-Al2O3-RF3-NaF Glasses: A Molecular Dynamics Simulation Study.

Oxyfluoride glass-ceramics with RF3 or NaRF4 (R3+: rare earth elements) nanocrystals are considered as favorable hosts for luminescence applications. In this work, we utilized large-scale molecular dynamics (MD) simulations with effective partial charge potentials to study a series of oxyfluoride glasses that are of interest to the precipitation of RF3 or NaRF4 nanocrystals as previous experiment results suggested. The results show that phase separation exists in all glass compositions with fluoride-rich regions made up of R3+, Na+, and F- and oxide-rich regions consisting of aluminosilicate networks. These fluoride-enriched regions can serve as the precursor for RF3, cubic and hexagonal NaRF4, and NaF crystal precipitation. The results also confirm that the concentration of Na+ in the fluoride phase plays a key role in determining the crystal phases (RF3, NaRF4, or NaF) and crystal structure (cubic vs hexagonal NaRF4) to be precipitated. Consequently, this study shows that MD simulations with effective potentials can fill the gap in the structural understanding of oxyfluoride glass and provide insights into atomic scale information of the phase separation behavior that is useful in predicting the potential crystal types in oxyfluoride glass. When coupled with experimental validations, these simulations can expedite the exploration of novel luminescent oxyfluoride glass ceramics.

Interfacial structures of spinel crystals with borosilicate nuclear waste glasses from molecular dynamics simulations

AbstractSpinel crystal formation presents a critical issue and glass formulation in nuclear waste glass processing. In this paper, the interfacial structures of the model borosilicate nuclear waste glasses, the international simple glass (ISG), with two types of spinel crystals, namely the MgAl2O4 and NiFe2O4, were studied using classical molecular dynamics simulations with effective partial charge potentials and recently developed composition‐dependent boron‐related parameters. The simulation results revealed the structural features of the borosilicate nuclear waste glasses and their interfaces with the two types of spinel crystals. It was found that there exist notable structural changes of glasses close to the interfacial region, affected by the adjacent crystal structures, terms of preferential segregation and ordering of cations, as well as ctaion coordination numbers. Specifically, the fraction of fourfold coordinated boron (B3) in glass near the interface decreases as compared to the bulk glass. In addition, the amount of fourfold coordinated Al decreases while fivefold Al increases in the glass region close to the glass‐crystal interface, which suggests indication of initial stage of crystal growth as Al adopts higher (sixfold) coordination like in the crystal as compared to majority of fourfold coordination in the glass. These interfacial structure changes obtained from MD simulations provide evidence of the influence of the precipitated crystals on the surrounding melt and glass and the initial stage of crystal growth.

An effective partial charge model for bulk and surface properties of cubic ZrO2, Y2O3 and yttrium-stabilised zirconia.

In this work a newly parametrised Coulomb plus Buckingham potential formulation for cubic ZrO2, Y2O3 and yttrium-stabilised zirconia (YSZ) is presented. The density and pair distributions obtained for neat ZrO2 and Y2O3 under ambient conditions are in excellent agreement with experimental data, while the vibrational power spectra are highly similar compared to those obtained via ab initio molecular dynamics simulations at the PBEsol level. In addition, it is shown that the use of effective partial charges has several advantages compared to interaction potentials employing the oxidation states in the evaluation of the coulombic interactions: (i) the diffusion coefficient and the associated activation energy of oxygen ions evaluated for YSZn (n = 4 to 12) display the best agreement with experimental data; (ii) no unphysical reorganisation of the interface and the bulk are observed in simulations of the (110) and (111) surfaces of cubic ZrO2 and Y2O3, while due to the strong coulombic contributions in the case of the tested full-charge models a pronounced restructuring of the interface and the bulk is observed in the ZrO2 case, and (iii) the use of effective partial charges ensures compatibility with existing solvent models and force-fields for the treatment of molecular compounds.

Fixed-Charge Atomistic Force Fields for Molecular Dynamics Simulations in the Condensed Phase: An Overview

In molecular dynamics or Monte Carlo simulations, the interactions between the particles (atoms) in the system are described by a so-called force field. The empirical functional form of classical fixed-charge force fields dates back to 1969 and remains essentially unchanged. In a fixed-charge force field, the polarization is not modeled explicitly, i.e. the effective partial charges do not change depending on conformation and environment. This simplification allows, however, a dramatic reduction in computational cost compared to polarizable force fields and in particular quantum-chemical modeling. The past decades have shown that simulations employing carefully parametrized fixed-charge force fields can provide useful insights into biological and chemical questions. This overview focuses on the four major force-field families, i.e. AMBER, CHARMM, GROMOS, and OPLS, which are based on the same classical functional form and are continuously improved to the present day. The overview is aimed at readers entering the field of (bio)molecular simulations. More experienced users may find the comparison and historical development of the force-field families interesting.

Structure, energetics, and electronic properties of stacking fault defects in ilmenite-structured ZnTiO3

The stacking fault behaviors on ilmenite ZnTiO3 were investigated by calculating the generalized stacking fault (GSF) energies using density functional theory (DF T) based on first principles calculations and classical calculations employing effective partial charge interatomic potentials. The results show that stable and unstable stacking fault energies are in qualitative agreement and provide the same sequence of stacking fault energies, although there exist quantitative differences with DFT providing lower stable and unstable stacking fault energy values than those from classical potential calculations. The γ-surfaces of two low energy surfaces, (1 1 0) and (1 0 4), of ZnTiO3 were fully mapped together with ideal shear stress (ISS) calculations. It was found that stacking faults along 〈〉/{1 0 4} are preferred energetically and are most probable to nucleate dislocations due to their significantly lower γsf/ γusf values. The atomic structures of the low energy stacking faults were analyzed and their electronic structures calculated and compared with bulk ZnTiO3 structures. It was found that stacking fault formation led to narrowing of the band gap and creating inter-bandgap states, mainly due to the dangling bonds and bonding defects, as compared to the bulk structures.

Experimental and computational studies on stacking faults in zinc titanate

Zinc titanate (ZnTiO3) thin films grown by atomic layer deposition with ilmenite structure have recently been identified as an excellent solid lubricant, where low interfacial shear and friction are achieved due to intrafilm shear velocity accommodation in sliding contacts. In this Letter, high resolution transmission electron microscopy with electron diffraction revealed that extensive stacking faults are present on ZnTiO3 textured (104) planes. These growth stacking faults serve as a pathway for dislocations to glide parallel to the sliding direction and hence achieve low interfacial shear/friction. Generalized stacking fault energy plots also known as γ-surfaces were computed for the (104) surface of ZnTiO3 using energy minimization method with classical effective partial charge potential and verified by using density functional theory first principles calculations for stacking fault energies along certain directions. These two are in qualitative agreement but classical simulations generally overestimate the energies. In addition, the lowest energy path was determined to be along the [451¯] direction and the most favorable glide system is {104} ⟨451¯⟩ that is responsible for the experimentally observed sliding-induced ductility.

Atomic partial charges in condensed phase from an exact sum rule for infrared absorption

A general sum rule for infrared intensities provides a definition of effective partial charges which can be experimentally obtained using isotopic substitutions and is valid in both gas and condensed phases. Of particular interest is the case of molecular liquids. We have, therefore, determined the hydrogen partial charges in liquid methanol and liquid water from the available literature. The resulting charges are 0.63 e and 0.14 e for hydrogen atoms bounded to the methanol oxygen and carbon atoms, respectively, and 0.55 e for hydrogen atoms in liquid water. The effective partial charges in liquid water were also computed from density functional based ab initio molecular dynamics simulations and found in good agreement with experiment.

Effect of strontium substitution on the structure, ionic diffusion and dynamic properties of 45S5 Bioactive glasses

Strontium ions can promote bone growth and enhance bioactivity in bioactive glasses that find critical biomedical applications. In this paper, the effect of SrO/CaO substitution on the structure, ionic diffusion, and dynamic properties of 45S5 bioactive glasses has been studied using constant pressure molecular dynamics simulations with a set of effective partial charge potentials. The simulated structure models were validated by comparing with experimental neutron diffraction results. It was found that the SrO/CaO substitution leads to an increase of glass density and decrease of oxygen density, a measure of compactness of the glass, in excellent agreement with available experimental data. On the other hand, the substitution does not significantly change of the medium range glass structures as characterized by silicon and phosphorous Qn distributions, network connectivity, and ring size distributions. The diffusion and dynamic behavior of these glasses and their melts were also determined by calculating the mean square displacements and velocity autocorrelation functions. It was found that the diffusion energy barriers of sodium, calcium and strontium ions remain nearly constant with respect to the level of substitution. However, strontium ions do influence the diffusion behaviors of calcium and sodium ions at high temperature, as evidenced from their velocity autocorrelation functions. Like calcium and sodium, strontium ions only contribute to the lower frequency (around 100cm−1) of the vibrational spectra the substitution has little effect on high frequency features and the general shape of the vibrational density of states. These results suggest that the increase of the dissolution rate in strontium containing glasses are mainly due to the increase of free volume and the non-local effect that weakens the silicon-oxygen network due to strontium ions.

Europium environment and clustering in europium doped silica and sodium silicate glasses

The local environment and distribution of rare earth ions are important to the optical properties of rare earth doped oxide glasses. In this paper, we report studies of the structures of europium doped (around 1 mol% Eu 2O 3) silica and sodium silicate glasses using molecular dynamics simulations. By using effective partial charge potentials, systems with over 24,000 atoms were modeled in order to obtain better statistics of rare earth ion distribution. The simulated glass structures were validated by comparing the calculated neutron and X-ray structure factors with experimental data. It was found that europium ions have higher coordination number (5.9 versus 4.8) and more symmetric environments in sodium silicate glasses than in the silica glass. Rare earth ion clustering has been characterized in detail and it was found that the clustering probability of europium ions in sodium silicate glass is consistently less than that predicted from a random distribution, while the probability of clustering in pure silica glass is higher than that of random distribution at the 1 mol% doping level.

Selective Adsorption of Oxygen over Argon in Alkaline-Earth-Metal Cation-Exchanged Zeolite X

The separation of argon and oxygen from their gaseous mixture by adsorption is challenging because of the closeness of their molecular properties and, hence, adsorption behavior. In the present study, the potential of zeolite X with strontium as the extraframework cation as an oxygen-selective adsorbent for the separation and purification of argon is discussed. Equilibrium adsorption isotherms of oxygen and argon on calcium, strontium, and barium cation-exchanged zeolite X were measured at 288 and 303 K. The equilibrium adsorption capacity and selectivity for oxygen over argon were observed to be higher in zeolite X exchanged with alkaline-earth-metal cations than in NaX. Among these alkaline-earth-metal cation-exchanged zeolite X adsorbents, SrX showed the highest adsorption selectivity, in the range of 2.0 at 50 mmHg to 1.8 at 760 mmHg for oxygen over argon at 303 K. The increased selectivity for oxygen over argon in SrX is discussed in terms of the size, location, and effective partial charges on the e...

Redox potential of the Rieske iron–sulfur protein: Quantum-chemical and electrostatic study

Quantum-chemical study of structures, energies, and effective partial charge distribution for several models of the Rieske protein redox center is performed in terms of the B3LYP density functional method in combination with the broken symmetry approach using three different atomic basis sets. The structure of the redox complex optimized in vacuum differs markedly from that inside the protein. This means that the protein matrix imposes some stress on the active site resulting in distortion of its structure. The redox potentials calculated for the real active site structure are in a substantially better agreement with the experiment than those calculated for the idealized structure. This shows an important role of the active site distortion in tuning its redox potential. The reference absolute electrode potential of the standard hydrogen electrode is used that accounts for the correction caused by the water surface potential. Electrostatic calculations are performed in the framework of the polarizable solute model. Two dielectric permittivities of the protein are employed: the optical permittivity for calculation of the intraprotein electric field, and the static permittivity for calculation of the dielectric response energy. Only this approach results in a reasonable agreement of the calculated and experimental redox potentials.

Simulating CH4 physisorption on ionic crystals

We use quantum chemical techniques to evaluate the electrostatic and polarization components of the interaction between a rigid CH4 molecule and a lattice of point charges representing the MgO(100) surface. We find that CH4 positioned above Mg adopts an edge-down configuration in which two H atoms are oriented downward towards the MgO(100) surface and point at O ions in the surface layer. The CH4–MgO(100) electrostatic interaction is substantially less favorable (but is still attractive) for the face-down configuration in which three H atoms point downward. Neither configuration is energetically favorable for CH4 molecules positioned above O ions. We show that for edge-down CH4 molecules above Mg, the electrostatic component of the CH4-substrate interaction varies considerably as the CH4 molecule rotates about the surface normal; the polarization component of the interaction, by contrast, is nearly constant during this rotation. We show that a point-charge model for the CH4 charge distribution, in which the C and H atoms carry effective partial charges, predicts that the CH4-surface electrostatic interaction should be more favorable for face-down CH4 molecules than for edge-down CH4 molecules, in disagreement with the quantum chemical results. We show that this is because the point-charge model poorly represents the high-order electric multipoles of CH4.

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

The simple cut criterion based on the accurate determination of the radii of the ions in alkali halides and previously introduced by the authors for forecasting off-centre configuration of Li+ and F− has been extended to heavy ions (Ag+ and Cu+). It has been found that this criterion is valid for the Ag+ ion, whereas for Cu+ gives a less precise forecast because of the lack in knowledge of the effective partial charge on Cu+ ion. It has been evidenced that the critical value of the ratior + * /r + between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass.