Accuracy of buffered-force QM/MM simulations of silica

Ultrasmall-Scale Brittle Fracture Initiated from a Dislocation in SrTiO3

Crystal defects often lead to an intriguing variety of catastrophic failures of materials and determine the mechanical properties. Here we discover that a dislocation, which was believed to be a source of plasticity, leads to brittle fracture in SrTiO₃. The fracture mechanism, i.e., bond breaking at the dislocation core triggers crack initiation and subsequent fracture, is elucidated from an atomic view by hybrid quantum and molecular simulations and in situ nanomechanical experiments. The fracture strength of the dislocation-included SrTiO₃ was theoretically evaluated to be 8.8-10.7 GPa, which was eminently lower than that of the pristine one (21.7 GPa). The experimental results agree well with the simulated ones. Moreover, the fracture toughness of the ultrasmall crack initiating from the dislocation is quantitatively evaluated. This study reveals not only the role of dislocations in brittle fracture but also provides an in-depth understanding of the fracture mechanism of dislocations at the atomic scale.

On-the-fly determination of active region centers in adaptive-partitioning QM/MM

Quantum mechanics/molecular mechanics (QM/MM) methods are widely used in molecular dynamics (MD) simulations of large systems. By partitioning the system into active and environmental regions and treating them with different levels of theory, QM/MM methods achieve accuracy and efficiency at the same time. Adaptive-partitioning (AP) QM/MM allows the partition of the system to change during the MD simulation, making it possible to simulate processes in which the active and environmental regions exchange atoms or molecules, such as processes in solutions or solids. AP-QM/MM methods usually partition the system according to distances to centers of active regions. For energy-conserving AP-QM/MM methods, these centers are chosen beforehand and remain fixed during the MD simulation, making it difficult to simulate processes in which active regions may occur or vanish. In this paper, I develop an adaptive-center (AC) method that allows on-the-fly determination of the centers of active regions according to any geometrical criterion or any criterion dependent on the potential energy. The AC method is compatible with all existing energy-conserving AP-QM/MM methods, and the resulting potential energy surface is smooth. The application of the AC method is demonstrated with two examples in solid systems.

Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube–polymer composites

We use a QM/MM approach to simulate CNT pull-out from crosslinked polyethylene studying how interfacial strength depends on its chemistry.

Theoretical investigations of a platinum-water interface using quantum-mechanics-molecular-mechanics based molecular dynamics simulations

Pt-water interfaces have been of immense interest in the field of energy storage and conversion. Studying this interface using both experimental and theoretical tools is challenging. On the theoretical front, typically one uses classical molecular dynamics (MD) simulations to handle large system sizes or time scales while for a more accurate quantum mechanical description Born Oppenheimer MD (BOMD) is typically used. The latter is limited to smaller system sizes and time-scales. In this study using quantum-mechanics-molecular-mechanics (QMMM), we have performed atomistic MD simulations to have a microscopic understanding of the structure of the Pt-water interface using a system size that is much larger than that accessible when using BOMD simulations. In contrast to recent reports using BOMD simulations, our study reveals that the water molecules typically form two distinct layers above the Pt-surface before they form bulk like structures. Further, we also find that a significant fraction of the water molecules at the interface are pointed towards the surface thereby disrupting the H-bond network. Consistent with this observation, the layer resolved oxygen-oxygen radial distribution function for the water molecules belonging to the solvating water layer shows a high density liquid like behaviour even though the overall water behaves like a low density liquid. A charge transfer analysis reveals that this solvating water layer donates electrons to the Pt atoms in contact with it thereby resulting in the formation of an interface dipole that is pointing towards the surface. Our results suggest that, using QMMM-MD, on one hand it is possible to study more realistic models of solid-liquid interfaces that are inaccessible with BOMD, while on the other hand one also has access to information about such systems that are not obtained from conventional classical MD simulations.

CPMD/GULP QM/MM interface for modeling periodic solids: Implementation and its application in the study of Y-zeolite supported Rhn clusters

We report here the development of hybrid quantum mechanics/molecular mechanics (QM/MM) interface between the plane-wave density functional theory based CPMD code and the empirical force-field based GULP code for modeling periodic solids and surfaces. The hybrid QM/MM interface is based on the electrostatic coupling between QM and MM regions. The interface is designed for carrying out full relaxation of all the QM and MM atoms during geometry optimizations and molecular dynamics simulations, including the boundary atoms. Both Born-Oppenheimer and Car-Parrinello molecular dynamics schemes are enabled for the QM part during the QM/MM calculations. This interface has the advantage of parallelization of both the programs such that the QM and MM force evaluations can be carried out in parallel to model large systems. The interface program is first validated for total energy conservation and parallel scaling performance is benchmarked. Oxygen vacancy in α-cristobalite is then studied in detail and the results are compared with a fully QM calculation and experimental data. Subsequently, we use our implementation to investigate the structure of rhodium cluster (Rhn ; n = 2 to 6) formed from Rh(C2 H4 )2 complex adsorbed within a cavity of Y-zeolite in a reducible atmosphere of H2 gas. © 2016 Wiley Periodicals, Inc.