Bond-order potentials with split-charge equilibration: application to C-, H-, and O-containing systems.

A method for extending charge transfer to bond-order potentials, known as the bond-order potential/split-charge equilibration (BOP/SQE) method [P. T. Mikulski, M. T. Knippenberg, and J. A. Harrison, J. Chem. Phys. 131, 241105 (2009)], is integrated into a new bond-order potential for interactions between oxygen, carbon, and hydrogen. This reactive potential utilizes the formalism of the adaptive intermolecular reactive empirical bond-order potential [S. J. Stuart, A. B. Tutein, and J. A. Harrison, J. Chem. Phys. 112, 6472 (2000)] with additional terms for oxygen and charge interactions. This implementation of the reactive potential is able to model chemical reactions where partial charges change in gas- and condensed-phase systems containing oxygen, carbon, and hydrogen. The BOP/SQE method prevents the unrestricted growth of charges, often observed in charge equilibration methods, without adding significant computational time, because it makes use of a quantity which is calculated as part of the underlying covalent portion of the potential, namely, the bond order. The implementation of this method with the qAIREBO potential is designed to provide a tool that can be used to model dynamics in a wide range of systems without significant computational cost. To demonstrate the usefulness and flexibility of this potential, heats of formation for isolated molecules, radial distribution functions of liquids, and energies of oxygenated diamond surfaces are calculated.

Merging bond-order potentials with charge equilibration.

A method is presented for extending any bond-order potential (BOP) to include charge transfer between atoms through a modification of the split-charge equilibration (SQE) formalism. Variable limits on the maximum allowed charge transfer between atomic pairs are defined by mapping bond order to an amount of shared charge in each bond. Charge transfer is interpreted as an asymmetry in how the shared charge is distributed between the atoms of the bond. Charge equilibration (QE) assesses the asymmetry of the shared charge, while the BOP converts this asymmetry to the actual amount of charge transferred. When applied to large molecules, this BOP/SQE method does not exhibit the unrealistic growth of charges that is often associated with QE models.

Friction between solids.

The theoretical examination of the friction between solids is discussed with a focus on self-assembled monolayers, carbon-containing materials and antiwear additives. Important findings are illustrated by describing examples where simulations have complemented experimental work by providing a deeper understanding of the molecular origins of friction. Most of the work discussed herein makes use of classical molecular dynamics (MD) simulations. Of course, classical MD is not the only theoretical tool available to study friction. In view of that, a brief review of the early models of friction is also given. It should be noted that some topics related to the friction between solids, i.e. theory of electronic friction, are not discussed here but will be discussed in a subsequent review.

Odd and even model self-assembled monolayers: links between friction and structure.

The friction between an amorphous carbon tip and two n-alkane monolayers has been examined using classical molecular dynamics simulations. The two monolayers have the same packing density, but the chains comprising each monolayer differ in length by one -CH2- unit. The simulations show that the monolayers composed of C13 chains have higher friction than those composed of C14 chains when sliding in the direction of chain cant; the difference in friction becomes more pronounced as the load is increased. Examination of the contact forces between the chains and the tip, along with conformational differences between the two chain types, lends insight into the friction differences.

Effect of filling on the compressibility of carbon nanotubes: predictions from molecular dynamics simulations.

The compressibility of filled and empty (10, 10) carbon nanotubes (CNTs) is examined using classical molecular dynamics simulations. The filled nanotubes contain C60, CH4, Ne, n-C4H10, and n-C4H7 molecules that are covalently cross-linked to the inner CNT walls. In addition, nanotubes filled with either a hydrogen-terminated carbon nanowire or a carbon nanotube of comparable diameter is also considered. The forces on the atoms are calculated using a many-body reactive empirical bond-order potential and the adaptive intermolecular reactive empirical bond-order potential for hydrocarbons. The butane-filled system shows a unique yielding behavior prior to buckling that has not been observed previously. Cross-linking the molecules to the inner CNT walls is not predicted to affect the stiffness of the filled nanotube systems and removes the yielding response. The mechanical response of the nanowire filled CNT is remarkably similar to the response of the similarly sized multiwalled CNT.

Contact forces at the sliding interface: mixed versus pure model alkane monolayers.

Classical molecular dynamics simulations of an amorphous carbon tip sliding against monolayers of n-alkane chains are presented. The tribological behavior of tightly packed, pure monolayers composed of chains containing 14 carbon atoms is compared to mixed monolayers that randomly combine equal amounts of 12- and 16-carbon-atom chains. When sliding in the direction of chain cant under repulsive (positive) loads, pure monolayers consistently show lower friction than mixed monolayers. The distribution of contact forces between individual monolayer chain groups and the tip shows pure and mixed monolayers resist tip motion similarly. In contrast, the contact forces "pushing" the tip along differ in the two monolayers. The pure monolayers exhibit a high level of symmetry between resisting and pushing forces which results in a lower net friction. Both systems exhibit a marked friction anisotropy. The contact force distribution changes dramatically as a result of the change in sliding direction, resulting in an increase in friction. Upon continued sliding in the direction perpendicular to chain cant, both types of monolayers are often capable of transitioning to a state where the chains are primarily oriented with the cant along the sliding direction. A large change in the distribution of contact forces and a reduction in friction accompany this transition.

Molecular-scale tribology of amorphous carbon coatings: effects of film thickness, adhesion, and long-range interactions.

Classical molecular dynamics simulations have been conducted to investigate the atomic-scale friction and wear when hydrogen-terminated diamond (111) counterfaces are in sliding contact with diamond (111) surfaces coated with amorphous, hydrogen-free carbon films. Two films, with approximately the same ratio of sp(3)-to-sp(2) carbon, but different thicknesses, have been examined. Both systems give a similar average friction in the load range examined. Above a critical load, a series of tribochemical reactions occur resulting in a significant restructuring of the film. This restructuring is analogous to the "run-in" observed in macroscopic friction experiments and reduces the friction. The contribution of adhesion between the probe (counterface) and the sample to friction was examined by varying the saturation of the counterface. Decreasing the degree of counterface saturation, by reducing the hydrogen termination, increases the friction. Finally, the contribution of long-range interactions to friction was examined by using two potential energy functions that differ only in their long-range forces to examine friction in the same system.

Compression of carbon nanotubes filled with C60, CH4, or Ne: predictions from molecular dynamics simulations.

The effect of filling nanotubes with C60, CH4, or Ne on the mechanical properties of the nanotubes is examined. The approach is classical molecular dynamics using the reactive empirical bond order (REBO) and the adaptive intermolecular REBO potentials. The simulations predict that the buckling force of filled nanotubes can be larger than that of empty nanotubes, and the magnitude of the increase depends on the density of the filling material. In addition, these simulations demonstrate that the buckling force of empty nanotubes depends on temperature. Filling the nanotube disrupts this temperature effect so that it is no longer present in some cases.