Molecular mechanics and molecular dynamics analysis of Drexler-Merkle gears and neon pump

Over the past two years at the Materials and Process Simulation Center, we have been developing simulation approaches for studying the molecular nanomachine designs pioneered by Drexler and Merkle. These nanomachine designs, such as planetary gears and neon pump, are described with atomistic details and involve up to 10 000 atoms. With the Dreiding and universal force fields, we have optimized the structures of the two planetary gear designs and the neon pump. At the Fourth Foresight conference, we reported rotational impulse dynamics studies of the first and second generation designs of planetary gears undergoing very high-frequency rotational motions. We will explore stability of these designs in the lower frequency regimes which require long time simulations. We will report the molecular mechanics and molecular dynamics simulations performed on these model systems. We explore the following modes in these studies: (1) impulse mode; (2) constant angular velocity - perpetual rotation; (3) constant torque - acceleration from rest

Nonadiabatic Study of Dynamic Electronic Effects during Brittle Fracture of Silicon

It has long been observed that brittle fracture of materials can lead to emission of high energy electrons and UV photons, but an atomistic description of the origin of such processes has lacked. We report here on simulations using a first-principles-based electron force field methodology with effective core potentials to describe the nonadiabatic quantum dynamics during brittle fracture in silicon crystal. Our simulations replicate the correct response of the crack tip velocity to the threshold critical energy release rate, a feat that is inaccessible to quantum mechanics methods or conventional force-field-based molecular dynamics. We also describe the crack induced voltages, current bursts, and charge carrier production observed experimentally during fracture but not previously captured in simulations. We find that strain-induced surface rearrangements and local heating cause ionization of electrons at the fracture surfaces

General Multiobjective Force Field Optimization Framework, with Application to Reactive Force Fields for Silicon Carbide

First-principles-based force fields prepared from large quantum mechanical data sets are now the norm in predictive molecular dynamics simulations for complex chemical processes, as opposed to force fields fitted solely from phenomenological data. In principle, the former allow improved accuracy and transferability over a wider range of molecular compositions, interactions, and environmental conditions unexplored by experiments. That is, assuming they have been optimally prepared from a diverse training set. The trade-off has been force field engines that are functionally complex, with a large number of nonbonded and bonded analytical forms that give rise to rather large parameter search spaces. To address this problem, we have developed GARFfield (genetic algorithm-based reactive force field optimizer method), a hybrid multiobjective Pareto-optimal parameter development scheme based on genetic algorithms, hill-climbing routines and conjugate-gradient minimization. To demonstrate the capabilities of GARFfield we use it to develop two very different force fields: (1) the ReaxFF reactive force field for modeling the adiabatic reactive dynamics of silicon carbide growth from an methyltrichlorosilane precursor and (2) the SiC electron force field with effective core pseudopotentials for modeling nonadiabatic dynamic phenomena with highly excited electronic states. The flexible and open architecture of GARFfield enables efficient and fast parallel optimization of parameters from quantum mechanical data sets for demanding applications like ReaxFF, electronic fast forward (or electron force field), and others including atomistic reactive charge-optimized many-body interatomic potentials, Morse, and coarse-grain force fields

Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials

The fundamental processes in shock-induced instabilities of materials remain obscure, particularly for detonation of energetic materials. We simulated these processes at the atomic scale on a realistic model of a polymer-bonded explosive (3,695,375 atoms/cell) and observed that a hot spot forms at the nonuniform interface, arising from shear relaxation that results in shear along the interface that leads to a large temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that lead to detonation. We show that decreasing the density of the binder eliminates the hot spot

Development of a ReaxFF Reactive Force Field for Ettringite and Study of its Mechanical Failure Modes from Reactive Dynamics Simulations

Ettringite is a hexacalcium aluminate trisulfate hydrate mineral that forms during Portland cement hydration. Its presence plays an important role in controlling the setting rate of the highly reactive aluminate phases in cement paste and has also been associated with severe cracking in cured hardened cement. To understand how it forms and how its properties influence those of hardened cement and concrete, we have developed a first-principles-based ReaxFF reactive force field for Ca/Al/H/O/S. Here, we report on the development of this ReaxFF force field and on its validation and application using reactive molecular dynamics (RMD) simulations to characterize and understand the elastic, plastic, and failure response of ettringite at the atomic scale. The ReaxFF force field was validated by comparing the lattice parameters, pairwise distribution functions, and elastic constants of an ettringite crystal model obtained from RMD simulations with those from experiments. The predicted results are in close agreement with published experimental data. To characterize the atomistic failure modes of ettringite, we performed stress–strain simulations to find that Ca–O bonds are responsible for failure of the calcium sulfate and tricalcium aluminate (C3A) column in ettringite during uniaxial compression and tension and that hydrogen bond re-formation during compression induces an increase in plastic strain beyond the material’s stress–strain proportionality limit. These results provide essential insight into understanding the mechanistic role of this mineral in cement and concrete degradation, and the ReaxFF potential developed in this work serves as a fundamental tool to further study the kinetics of hydration in cement and concrete

Large-scale Molecular Simulations of Hypervelocity Impact of Materials

We describe the application of the ReaxFF reactive force field with short-range distance-dependent exponential inner wall corrections and the non-adiabatic electron Force Field (eFF) for studying the hypervelocity impact (HVI) effects on material properties. In particular, to understanding nonequilibrium energy/mass transfer, high strain/heat rate material decomposition, defects formation, plastic flow, phase transitions, and electronic excitation effects that arise from HVI impact of soft and hard materials on different material surfaces. Novel results are presented on the single shock Hugoniot and shock chemistry of Nylon6-6, on the hypervelocity shock sensitivity of energetic materials with planar interfacial defects and on HVI chemistry of silicon carbide surfaces with diamondoid nanoparticles. Both methods provide a means to elucidate the chemical, atomic and molecular processes that occur within the bulk and at the surfaces of materials subjected to HVI conditions and constitute a critical tool to enabling technologies required for the next generation of energy, spatial, transportation, medical, and military systems and devices, among many others. This has proven to be extremely challenging, if not impossible, for experimental observations, mainly because the material states that occur are hard to isolate and their time scales for changes are too rapid (<1 ps). First principles quantum mechanics (QM) simulation methods have also been bounded by the prohibitive scaling cost of propagating the total Schrödinger equation for more than 100 atoms at finite temperatures and pressures

First principles-based multiscale atomistic methods for input into first principles nonequilibrium transport across interfaces

This issue of PNAS features “nonequilibrium transport and mixing across interfaces,” with several papers describing the nonequilibrium coupling of transport at interfaces, including mesoscopic and macroscopic dynamics in fluids, plasma, and other materials over scales from microscale to celestial. Most such descriptions describe the materials in terms of the density and equations of state rather than specific atomic structures and chemical processes. It is at interfacial boundaries where such atomistic information is most relevant. However, there is not yet a practical way to couple these phenomena with the atomistic description of chemistry. The starting point for including such information is the quantum mechanics (QM). However, practical QM calculations are limited to a hundred atoms for dozens of picoseconds, far from the scales required to inform the continuum level with the proper atomistic description. To bridge this enormous gap, we need to develop practical methods to extend the scale of the atomistic simulation by several orders of magnitude while retaining the level of QM accuracy in describing the chemical process. These developments would enable continuum modeling of turbulent transport at interfaces to incorporate the relevant chemistry. In this perspective, we will focus on recent progress in accomplishing these extensions in first principles-based atomistic simulations and the strategies being pursued to increase the accuracy of very large scales while dramatically decreasing the computational effort

The atomistic origin of the extraordinary oxygen reduction activity of Pt_3Ni_7 fuel cell catalysts

Recently Debe et al. reported that Pt_3Ni_7 leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface (“Pt-skin effect”). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt_3Ni_7 while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt–Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ~300 000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt_3Ni_7 case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the O_(ad)-hydration reaction (O_(ad) + H_2O_(ad) → OH_(ad) + OH_(ad)) but including the barriers of O_2 dissociation (O_(2ad) → O_(ad) + O_(ad)) and water formation (OH_(ad) + H_(ad) → H_2O_(ad)). We find that the reaction barrier for the O_(ad)-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt–Ni 30 : 70 initial composition. These simulations explain the peak in surface reactivity at Pt_3Ni_7, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials

Hypervelocity Impact Effect of Molecules from Enceladus’ Plume and Titan’s Upper Atmosphere on NASA’s Cassini Spectrometer from Reactive Dynamics Simulation

The NASA/ESA Cassini probe of Saturn analyzed the molecular composition of plumes emanating from one of its moons, Enceladus, and the upper atmosphere of another, Titan. However, interpretation of this data is complicated by the hypervelocity (HV) flybys of up to ∼18  km/sec that cause substantial molecular fragmentation. To interpret this data we use quantum mechanical based reactive force fields to simulate the HV impact of various molecular species and ice clathrates on oxidized titanium surfaces mimicking those in Cassini’s neutral and ion mass spectrometer (INMS). The predicted velocity dependent fragmentation patterns and composition mixing ratios agree with INMS data providing the means for identifying the molecules in the plume. We used our simulations to predict the surface damage from the HV impacts on the INMS interior walls, which we suggest acts as a titanium sublimation pump that could alter the instrument’s readings. These results show how the theory can identify chemical events from hypervelocity impacts in space plumes and atmospheres, providing in turn clues to the internal structure of the corresponding sources (e.g., Enceladus). This may be valuable in steering modifications in future missions

Predicted Detonation Properties at the Chapman-Jouguet State for Proposed Energetic Materials (MTO and MTO3N) from Combined ReaxFF and Quantum Mechanics Reactive Dynamics

The development of new energetic materials (EMs) with improved detonation performance but low sensitivity and environmental impact is of considerable importance for applications in civilian and military fields. Often new designs are difficult to synthesize so predictions of performance in advance is most valuable. Examples include MTO (2,4,6-triamino-1,3,5-triazine-1,3,5-trioxide) and MTO3N (2,4,6-trinitro-1,3,5-triazine-1,3,5-trioxide) suggested by Klapötke as candidate EMs but not yet successfully synthesized. We propose and apply to these materials a new approach, RxMD(cQM), in which ReaxFF Reactive Molecular Dynamics (RxMD) is first used to predict the reaction products and thermochemical properties at the Chapman Jouguet (CJ) state for which the system is fully reacted and at chemical equilibrium. Quantum mechanics dynamics (QMD) is then applied to refine the pressure of the ReaxFF predicted CJ state to predict a more accurate final CJ point, leading to a very practical calculation that includes accurate long range vdW interactions needed for accurate pressure. For MTO, this RxMD(cQM) method predicts a detonation pressure of PCJ = 40.5 GPa and a detonation velocity of DCJ = 8.8 km/s, while for MTO3N it predicts PCJ = 39.9 GPa and DCJ = 8.4 km/s, making them comparable to HMX (PCJ = 39.5 GPa, DCJ = 9.1 km/s) and worth synthesizing. This first-principles-based RxMD(cQM) methodology provides an excellent compromise between computational cost and accuracy including the formation of clusters that burn too slowly, providing a practical mean of assessing detonation performances for novel candidate EMs. This RxMD(cQM) method that links first principles atomistic molecular dynamics simulations with macroscopic properties to promote in silico design of new EMs should also be of general applicability to materials synthesis and processing