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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

A crystal plasticity finite element model embedding strain-rate sensitivities inherent to deformation mechanisms: Application to alloy AZ31

We report the fundamental power-law relationship representing the flow rule in crystal visco-plasticity ensures uniqueness in the selection of slip systems accommodating imposed plastic strain-rates. The power-law relationship also introduces an artificially high strain-rate sensitivity in crystal plasticity simulations, unless a high value of the power-law exponent is used. However, the use of high values for the exponent is limited by numerical tractability. This paper presents a numerical method implemented in a crystal plasticity finite element (CPFE) model for embedding any value of the power-law exponent reflecting the true material strain-rate sensitivity. Importantly, the method does not increase computation time involved in the simulations. The enhanced CPFE model is used to interpret and predict a complex strain-rate sensitive response and microstructural evolution of AZ31 Mg alloy. Measured values of strain-rate sensitivity for slip and twinning modes are used in the simulations. Calculations show that the model successfully captures the phenomena pertaining to the effect of changing applied strain-rate on the mechanical response including flow stress and evolution of texture and twinning for a broad range of strain-rates ranging from 10 -3 s -1 to 10 3 s -1 and loading orientations in tension and compression. It is shown that such predictions are a consequence of not only relative amounts of slip and twinning activities driven by a set of accurately characterized hardening law parameters but also values of the strain-rate sensitivities inherent to individual deformation mechanisms. Besides, the model validates the measured strain-rate dependency of deformation mechanisms while accurately reproducing the mechanical data. Hence, the model can be used to verify and further refine or infer measured strain-rate sensitivity per deformation mechanism by reproducing experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Modeling supercritical CO2 injection induced rupture of a minor fault embedded in a poroelastic layered reservoir-caprock system

CO2 injection for geologic carbon sequestration involves hydromechanical processes that lead to changes in fluid pressure and stresses that can activate existing faults. This paper presents a new method and workflow of modeling fault activation considering more complex three-dimensional geometry of natural faults using the TOUGH-FLAC multiphase fluid flow and geomechanical simulator. In this method and workflow, FLAC3D mechanical interfaces and TOUGH3 finite volume elements are discretized using computer aided design and gridding software along with a tailored mesh translation routine. The method and workflow are demonstrated with a model of a curved minor fault embedded in a poro-elastic layered reservoir-caprock system. The model is used for a comprehensive sensitivity analysis of fault responses to fault length, injection mass rate, injection schedule, well-fault distance, and well locations versus fault location. Four metrics (CO2 plume, shear state of fault, pressure and stress path at fault monitoring points) are selected to assess CO2 migration, pressure change, and the reactivation of faults. The results reveal that CO2 can bypass around the tip of the minor impermeable fault, building up pressure and poro-elastic stress on both sides that tends to impede fault rupture. Our study shows the benefit of carefully designing the injection to achieve the targeted final storage volume, starting at a relatively low rate for considerable time, and then ramping up the injection rate to the full rate of injection. The initial low injection has two distinct benefits: (1) it allows for the formation of an extensive CO2 plume with a much higher mobility through a low viscosity that will result in a lower pressure for a given injection rate, and (2) it allows for gradual build-up of horizontal poro-elastic stress within the reservoir that will tend to impede activation of steeply dipping faults. The injection scenario starting at a low injection rate, denoted here as conservative injection, can significantly reduce the risk of fault activation as high fluid mobility and reservoir strengthening poro-elastic stress has been established long before reaching the peak injection rates. Moreover, simultaneous injection in two injection wells on both sides of fault can provide further reservoir strengthening through poro-elastic stress buildup acting on a fault under normal faulting stress regime. The findings presented in the paper can provide practical and effective guidance on long-term, safe, and reliable geological CO2 storage.

Cao, Meng↗
DOE OSTI2020

Large elastic strains and ductile necking of W nanowires embedded in TiNi matrix

The deformation behaviors of W nanowires embedded in a TiNi matrix were investigated by means of in-situ synchrotron high energy X-ray diffraction (HEXRD) and in-situ transmission electron microscopy (TEM) analysis during tensile deformation. The HEXRD measurement indicated that the W nanowires exhibited an average lattice strain of about 1.50 %, whereas the TEM examination revealed a local elastic strain of about 4.59 % in areas adjacent to the TiNi matrix where stress-induced martensitic transformation occurred. This strain corresponds to a stress of ~15 GPa for the W nanowires. In addition, in areas adjacent to the TiNi matrix where plastic deformation and cracking were generated, the W nanowire showed significant ductile necking with ~80 % reduction in cross-section area. In conclusion, the ductile necking of W nanowire is attributed to the lack of protection from the stress-induced martensitic transformation of the TiNi matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Multifunctional properties of CoNi alloy embedded in the SiO{sub 2} host: Role of interparticle interaction

Nanocrystalline CoNi alloy is synthesized using sol-gel technique having average size {approx}8.5 and {approx}12nm. High-resolution transmission electron microscopy confirms absence of any oxide phase at the grain boundary. We observe multifunctionality ascribed to large loop shift after the field-cooling and saturation magnetization (M{sub S}) depending on interparticle interaction in CoNi alloy embedded in SiO{sub 2} matrix. A large loop shift 1.2 kOe is noticed at 5 K for 10.5% volume fraction ({phi}). The loop shift decreases with increasing temperature and vanishes around {approx}210K where a maximum is observed in zero-field-cooled magnetization. The loop shift decreases with decreasing {phi} and vanishes at {phi}=0.01%. Interestingly, M{sub S} increases with decreasing {phi} and value at {phi}=0.01% approaches to the calculated value of M{sub S} for the bulk counterpart. A model has been proposed with ferromagnetic-core and disordered magnetic-shell structure to interpret the results. - Graphical abstract: Plot of loop shift (left axis) and saturation magnetization (right axis) with volume fraction ({phi}). Inset: loop shift after field-cooling with 50 kOe field and no shift after zero-field-cooling. Highlights: > Strong exchange bias (EB) effect is observed for 10.5% volume fraction. > EB decreases with decreasing volume fraction ({phi}) and vanishes at {phi}=0.01%. > In contrast, saturation magnetization increases with decreasing volume fraction. > Saturation magnetization (M{sub S}) approaches to bulk value at 0.01% volume fraction. > Large EB and M{sub S} at different {phi} are appealing for multifunctional applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Experimental study of vibration isolation in thin-walled structural assemblies with embedded total-internal-reflection metasurfaces

The concept of total-internal-reflection elastic metasurface (TIR-MS) was recently proposed [1] and employed within flexible planar waveguides in order to create highly subwavelength sound-hard barriers impenetrable to low frequency elastic waves. The underlying physical mechanism relies on the design of engineered interfaces exhibiting extreme phase gradients such that any incoming wave at, approximately, any incidence will experience total-internal-reflection conditions. At the design frequency, the metasurface exhibits a large phase gradient such that, in accordance with the generalized Snell's law, the first critical angle is virtually always exceeded. It is worth noting that in practical realizations, the actual total reflection performance might vary depending on the angle of incidence. This dependence is due to the discrete implementation of the metasurface which results in diffraction effects. This paper presents the results of an experimental study that explores the vibration isolation performance of TIR-MS when applied to structures made of complex combinations of different elastic waveguides (e.g. bolted assemblies of beams, plates, and shells). Such system can be seen as a prototypical structure emulating mechanical assemblies of practical interest for many engineering applications. Experimental results confirm that, when the TIR-MS is embedded in the host waveguide, significant vibration isolation capabilities are achieved under quasi-omnidirectional incidence and highly subwavelength excitation conditions (i.e. the ratio of the operating wavelength to the width of the TIR-MS is approximately 5.25). Furthermore, these experimental results suggest new interesting directions to achieve vibration isolation and mechanical energy filtering for practical engineering systems.

42 ENGINEERING↗
DOE OSTI2018

Thermal actuation in $\mathrm{TRPV1}$: Role of embedded lipids and intracellular domains

The transient response potential cation channel TRPV1 responds to high temperature, but many of the mechanisms driving its thermal actuation remain unclear. Its recently resolved structure has enabled a number of molecular dynamics (MD) studies focused on illuminating these mechanisms. Here we add to these efforts by performing the first all-atom MD simulations of its most recently resolved structure at different temperatures. While the complete, thermally induced transition of TRPV1 from its closed to open configuration remains elusive, our analysis of the hydrogen bonding networks, thermodynamics, hydration, and principal components of motion provide a wealth of information on the mechanisms which initiate or influence the thermal opening in TRPV1. In particular, we (i) support the previously proposed mechanism driving thermal actuation in the extracellular pore of TRPV1, (ii) present new hypotheses regarding the thermal actuation in the C-terminal and adjacent linker domains, and (iii) support and build upon the existing hypothesis regarding the role of the vanilloid binding pocket and lipids embedded therein.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Embedding sulfur into N-doped carbon nanospheres as enhanced cathode for high-performance lithium-sulfur batteries

Graphical abstract: S has been successfully embedded into N-doped carbon nanospheres as enhanced cathode exhibiting good performance with high specific capacity and cycling stability. - Highlights: • Construct N-doped carbon nanospheres (N-CNSs) are host for sulfur. • N-CNSs/S cathode show high capacity and good cycles. • N doping is favorable for fast ion/electron transfer. - Abstract: Lithium-sulfur (Li-S) batteries are considered as one of the most efficient power candidates owing to high energy density about 7–10 times larger than the commercial cathode materials. However, its practical application is impeded by severe “shuttle effect” and low cycle stability. As a consequence, in this work, we report N-doped carbon nanospheres (N-CNSs) composited with S as cathode for Li-S batteries. The stacked carbon nanosphere structure can not only improve the conductivity of cathode, but also work as barriers to retard diffusion of intermediate polysulfide. Owing to the unique structure, the N-CNSs/S cathode exhibits superior electrochemical performances with high specific capacity (1228 mAh g{sup −1} at 0.1 C), and excellent rate capability as well as good cyclic life. Our smart designed strategy may promote the development of high performance Li-S batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of adjusting composition on the crystallization and luminescence properties in NaYF{sub 4}:Er{sup 3+} embedded glass ceramics

Highlights: • Intense 2.7 μm emission was achieved in transparent glass ceramics containing NaYF{sub 4}:Er{sup 3+} nanocrystals. • The crystallization and luminescence properties of the nanocrystals embedded glass ceramics are tuned by adjusting the Na{sub 2}O/NaF ratio. • Modifying the properties of glass ceramics through adjusting the glass composition is an effective route. - Abstract: Intense 2.7 μm emission was achieved in transparent glass ceramics containing NaYF{sub 4}:Er{sup 3+} nanocrystals. The effect of adjusting Na{sub 2}O/NaF ratio in glass composition on the crystallizing and luminescence behavior in the glass ceramics was investigated. The controllable crystallization property of NaYF{sub 4} nanocrystals was remained after modifying the glass composition and the crystallization temperature was largely reduced for the samples with high Na{sub 2}O content. The X-ray diffraction and transmission electron microscopy analysis evidenced the shrink of lattice parameter with increasing Na{sub 2}O content. High transparency was kept for these glass ceramic samples, and intense upconversion, near- and mid-infrared emissions were obtained in Er{sup 3+}-doped glass ceramics with an increase of red/green ratio and prominent Stark splits in the sample with high Na{sub 2}O content, indicating the change of crystal field around Er{sup 3+} in the host matrix. Our results indicate that the crystallization and luminescence properties of glass ceramics could be modified through adjusting the glass composition, and it is an effective route to develop glass ceramics with favorable properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Nanoparticle-embedded nanofibers in highly permselective thin-film nanocomposite membranes for forward osmosis

Here we report a scalable approach to fabricate osmotic membranes with high permselectivity based on nanocomposite of mesoporous silica nanoparticles and nanofibers by electrospinning. Transmission electron microscopy (TEM) images provide visualization of dispersed and clustered nanoparticles embedding within or at the surface of nanofibers. Energy-dispersive X-ray (EDX) point analysis confirms the chemical identity of the nanocomposite structure. Brunauer-Emmett-Teller (BET) analyses show a 75-fold increase in specific surface area when 15% of silica nanoparticles were integrated into polyacrylonitrile nanofibrous mats. Mechanical strength tests show that even at high load of silica nanoparticles, e.g. 15 wt%, the mechanical integrity of the membranes was maintained. Incorporating nanoparticles into nanofibrous mats enhanced their water uptake up to two times. In osmotic transport studies, we observed an outstanding permselectivity of our membranes compared to ones reported in literature. Our membranes show a remarkable 7-fold and 3.5-fold enhancements in osmotic water permeability and water/sodium chloride selectivity, respectively, compared to standard commercial forward osmosis membranes. These results suggest a pathway to develop scalable, high performance osmotic membranes and to further study the predominant mechanism governing transport behaviors of water and solute across nanomaterials interfaces.

42 ENGINEERING↗
DOE OSTI2018

A hybrid embedded discrete fracture model for simulating tight porous media with complex fracture systems

Numerical simulation considering complex fracture networks in tight oil reservoir becomes a hot issue during these years. Recently, embedded discrete fracture model (EDFM) shows great advantages over the DFM approaches in both calculation efficiency and simplicity of simulation workflow. EDFM has been widely used in tight oil/gas natural energy or enhanced oil recovery (EOR) development. If a large number of small-scale natural fractures exist in reservoir, the calculation efficiency of EDFM method may decrease. A new hybrid simulation model combining the EDFM and dual porosity (DP) model is proposed. The large-scale fractures are dealt with EDFM method while small scale fractures are dealt with DP model. Considering the low permeability of matrix, transient transfer between matrix and natural fracture is considered. The proposed hybrid model is validated compared with the local grid refinement (LGR) model and EDFM-MINC model, and the difference between transient transfer and pseudo-steady-state transfer effect on production is presented. Finally, two test cases are presented, and the sensitivity of some key parameters are investigated. Result shows that the new simulation model has higher calculation efficiency than LGR and EDFM-MINC models. Meanwhile, it honors a good accuracy. The pseudo-steady-state transfer between matrix and natural fracture leads to big error in tight oil production performance analysis both in single phase and two phases flow. Low matrix permeability and big matrix size increase the production difference between pseudo-steady-state transfer and transient transfer models. The proposed model can simulate reservoirs considering stochastic permeability distribution.

58 GEOSCIENCES↗
DOE OSTI2025

Pore Resolved Simulations of Joule Heating in Fibrous Media using an Embedded Boundary Method

Joule heating has been regarded as an energy-efficient and sustainable method for heating materials and gases at large scales. The modeling of local temperature effects at pore-resolved scales for such systems, however, has been difficult to achieve due to challenges in coupling thermo-chemical processes in complex porous media and in large representative volume elements (RVEs). To this end, we developed an electro-thermal model at the pore scale to study Joule heating effects in large heterogeneous systems with different microstructures. This was achieved using the level set method to implicitly delineate distinct regions within the domain, and an embedded boundary method to facilitate heat exchange across the fluid-solid interface. Moreover, we applied this method to investigate unsteady non-linear electro-thermal effects in non-woven fibrous graphite conductors for RVEs with characteristic lengths of 2 mm, with different fiber orientations, porosity (80% – 90%) and fiber diameters (10 – 20µm). The coupled equations were solved numerically and they produced peak temperatures greater than 2000 K resulting in heating rates as high as 80,000 K/s. Moreover, the results depended strongly on the microstructure of the fiber skeleton and current density. Geometries with large fibers (∼ 20µm) had the highest average and peak temperatures with the mean temperature increasing by 3.9 % while the peak temperature increased by 9.9 %. Anisotropic domains on the other hand had the lowest mean and peak temperatures with peak and mean temperatures of 2293 K and 1437.7K respectively representing a corresponding 12.1% and 5.1% drop in the temperatures. An increase in porosity from 80% to 90%, however, led to an increase in the peak temperature by 5.1%.

Joule heating↗
DOE OSTI2019

Dilute Pd/Au Alloy Nanoparticles Embedded in Colloid-Templated Porous SiO 2 : Stable Au-Based Oxidation Catalysts

Dilute alloy materials hold great promise for enhancing selectivity in catalytic reactions. A major challenge with such catalytic materials is developing supported alloy nanoparticles (NPs) with a catalytically optimal composition that are stable with respect to sintering. Here, we demonstrate that NP Pd/Au catalysts with minor concentrations of Pd prepared by a colloid-templating approach are active for oxygen dissociation and catalytic oxidation, as exemplified by CO oxidation. Using electron microscopic imaging, infrared spectroscopy, and X-ray photoelectron spectroscopy, we demonstrate that Pd is stabilized within the Au nanoparticle in its metallic state with minor amounts of Pd at the surface. The Pd–Au nanoparticles are embedded in the colloidally templated silica and form a thermally stable catalyst at high temperature. Surface analysis by diffuse reflectance infrared spectroscopy indicates that the Pd atomic distribution at the surface varies with the bulk concentration, forming isolated Pd atoms at 2% Pd and a mixture of Pd monomers and small Pd clusters at 9% Pd. X-ray absorption fine structure analysis indicates that Pd is well distributed in the bulk of the nanoparticle. In conclusion, in-depth understanding of this new class of catalytic materials opens the way to a wide array of possibilities for crucial enhancement of activity and selectivity in catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Large-Scale First-Principles Molecular Dynamics Simulations with Electrostatic Embedding: Application to Acetylcholinesterase Catalysis

Enzymes are complicated solvated systems that typically require many atoms to simulate their function with any degree of accuracy. We have recently developed numerical techniques for large scale First-Principles molecular dynamics simulations and applied them to study the enzymatic reaction catalyzed by acetylcholinesterase. We carried out Density functional theory calculations for a quantum mechanical (QM) sub- system consisting of 612 atoms with an O(N) complexity finite-difference approach. The QM sub-system is embedded inside an external potential field representing the electrostatic effect due to the environment. We obtained finite temperature sampling by First-Principles molecular dynamics for the acylation reaction of acetylcholine catalyzed by acetylcholinesterase. Our calculations shows two energies barriers along the reaction coordinate for the enzyme catalyzed acylation of acetylcholine. In conclusion, the second barrier (8.5 kcal/mole) is rate-limiting for the acylation reaction and in good agreement with experiment.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Hierarchical Reinforcement Learning of a Short-Range Bond-Order Potential for Silica: Analytic Embedding of Coordination with Classical Efficiency

Reinforcement learning (RL) has recently emerged as a data-efficient strategy to parametrize short-range interatomic potentials. Building on our past RL optimization of pairwise silica models, we extend the framework to a bond-order (Tersoff-type) potential that provides an analytic embedding of local coordination through a three-body term. A hierarchical RL workflow combining continuous-action Monte Carlo Tree Search and property-based rewards efficiently explores the 26-dimensional parameter space, sequentially optimizing lattice parameters, densities, angles, and cohesive energies of 21 silica polymorphs. The resulting models, Q-Tersoff and ML-Tersoff, reproduce the energetic ordering of low-energy phases and capture the angular correlations and amorphous structure factors of silica with improved fidelity over pairwise force fields, while remaining orders of magnitude faster than high-dimensional machine-learned potentials. Both models underperform for elastic constants and high-energy frameworks, delineating the limits of the current analytic form. The approach establishes a general and interpretable route to angle-aware, short-range potentials that bridge physics-based and machine-learned descriptions of silicate materials.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Automatic Partition of Orbital Spaces Based on Singular Value Decomposition in the Context of Embedding Theories

Here, we present a simple approach for orbital space partitioning to be employed in the projection-based embedding theory developed by Goodpaster and co-workers [Manby et al. J. Chem. Theory Comput. 2012, 8, 2564]. Once the atoms are assigned to the desired subspaces, the molecular orbitals are projected onto the atomic orbitals centered on active atoms and then singular value decomposed. The right singular vectors are used to rotate the initial molecular orbitals, taking the largest gap in the singular values spectrum to define the most suitable partition of the occupied orbital space. This scheme is free from numerical parameters, contrary to the Mulliken charge threshold or the completeness criterion previously used. The performance of this new prescription is assessed in a test set of several distinct reactions, the deprotonation of decanoic acid, the Diels-Alder reaction of 1,3-butadiene and octadecanonaene, the torsional potential of a retinal derivative, and the critical points along the reaction coordinate of an instance of the Menshutkin S N2 reaction inside a carbon nanotube.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Charge Transport through Organic Molecular Wires Embedded in Ultrathin Insulating Inorganic Layer

Dense phase silica layers with thickness of a few nanometers featuring embedded organic molecular wires of type p-oligo(phenylenevinylene) are shown by visible light sensitized electrochemical measurements to transport charges across the insulating membrane. Here, we find that such hybrid materials combination allows electronic charge transport only through the wires, while blocking molecular transport. Embodiment of the wire molecules in the silica was accomplished by atomic layer deposition under mild temperature conditions. Grown on Co oxide films for water oxidation, with the wire molecules covalently anchored on the oxide surface, the layer functions as a proton conducting separation membrane. Characterization by XPS, FT-IR and STEM/EDX confirms the integrity of the silica-encapsulated organic wires. Cyclic voltammetry with redox couple of selected potential relative to the energy levels of the wire molecules shows that the membrane is free of pinholes. The new type of membrane allows separation of incompatible redox reaction environments on the length scale of nanometers while enabling controlled electron transport between them. Finally, this opens up the coupling of carbon dioxide reduction with water oxidation, the essential reactions of artificial photosynthesis, in an integrated nanoscale photosystem.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Development of a Modified Embedded Atom Force Field for Zirconium Nitride Using Multi-Objective Evolutionary Optimization

Zirconium nitride (ZrN) exhibits exceptional mechanical, chemical, and electrical properties, which make it attractive for a wide range of technological applications, including wear-resistant coatings, protection from corrosion, cutting/shaping tools, and nuclear breeder reactors. Despite its broad usability, an atomic scale understanding of the superior performance of ZrN, and its response to external stimuli, for example, temperature, applied strain, and so on, is not well understood. This is mainly due to the lack of interatomic potential models that accurately describe the interactions between Zr and N atoms. To address this challenge, we develop a modified embedded atom method (MEAM) interatomic potential for the Zr–N binary system by training against formation enthalpies, lattice parameters, elastic properties, and surface energies of ZrN (and, in some cases, also Zr3N4) obtained from density functional theory (DFT) calculations. The best set of MEAM parameters are determined by employing a multiobjective global optimization scheme driven by genetic algorithms. Our newly developed MEAM potential accurately reproduces structure, thermodynamics, energetic ordering of polymorphs, as well as elastic and surface properties of Zr–N compounds, in excellent agreement with DFT calculations and experiments. As a representative application, we employed molecular dynamics simulations based on this MEAM potential to investigate the atomic scale mechanisms underlying fracture of bulk and nanopillar ZrN under applied uniaxial strains, as well as the impact of strain rate on their mechanical behavior. These simulations indicate that bulk ZrN undergoes brittle fracture irrespective of the strain rate, while ZrN nanopillars show quasi-plasticity owing to amorphization at the crack front. The MEAM potential for Zr–N developed in this work is an invaluable tool to investigate atomic-scale mechanisms underlying the response of ZrN to external stimuli (e.g, temperature, pressure etc.), as well as other interesting phenomena such as precipitation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Variational Formulation of the Generalized Many-Body Expansion with Self-Consistent Charge Embedding: Simple and Correct Analytic Energy Gradient for Fragment-Based ab Initio Molecular Dynamics

The many-body expansion (MBE) and its extension to overlapping fragments, the generalized (G)MBE, constitute the theoretical basis for most fragment-based approaches for large-scale quantum chemistry. We reformulate the GMBE for use with embedding charges determined self-consistently from the fragment wave functions, in a manner that preserves the variational nature of the underlying self-consistent field method. As a result, the analytic gradient retains the simple “sum of fragment gradients” form that is often assumed in practice, sometimes incorrectly. This obviates (without approximation) the need to solve coupled-perturbed equations, and we demonstrate stable, fragment-based ab initio molecular dynamics simulations using this technique. Energy conservation fails when charge-response contributions to the Fock matrix are neglected, even while geometry optimizations and vibrational frequency calculations may yet be accurate. Stable simulations can be recovered by means of straightforward modifications introduced here, providing a general paradigm for fragment-based ab initio molecular dynamics.

74 ATOMIC AND MOLECULAR PHYSICS↗