Scanning force microscopies ͑SFM͒ are being routinely used to examine the mechanical and tribological properties of materials with the goal of obtaining information, such as Young's Moduli and shear strengths from the experimental data ͓Unertl, J. Vac. Sci. Technol. A 17, 1779 ͑1999͔͒. Analysis of data obtained from an SFM experiment typically requires the use of continuum mechanics models to extract materials properties. When applying these models care must be taken to ensure that the experimental conditions meet the requirements of the model being applied. For example, despite many successful applications of the Johnson-Kendall-Roberts ͑JKR͒ model to SFM data, it does not take into account the presence of a compliant layer on the sample surface. Recent AFM experiments that examined the friction of self-assembled monolayers ͑SAMs͒ have confirmed that friction versus load data cannot be fit by the JKR model. The authors suggest that the penetration of the SAM by the tip gives rise to an additional contribution to friction due to "plowing" ͓Flater et al., Langmuir 23, 9242 ͑2007͔͒. Herein, molecular-dynamics simulations are used to study atomic contact forces between a spherical tip in sliding contact with a SAM. These simulations show that different regions around the tip contribute in unanticipated ways to the total friction between the tip and the monolayer and allow for the number and location of monolayer atoms contributing friction to be determined. The use of atomic contact forces within the monolayer, instead of forces on the rigid tip layers, allows for the contributions to friction force ͑and load͒ to be deconvoluted into forces that resist ͑repel͒ and assist ͑attract͒ tip motion. The findings presented here yield insight into the AFM experiments of SAMs and may have important consequences for the adaptation of continuum contact models for the contact between a sphere and surface where penetration into the sample is possible.

Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferability. When selecting a potential energy function for use in molecular simulations, the accuracy, transferability, and computational speed must all be considered. In this focused review, some of the more commonly used potential energy functions for molecular simulations are reviewed with an eye toward presenting their general forms, strengths, and weaknesses.

Because all quantities associated with a given atom are known as a function of time, molecular dynamics simulations can provide unparalleled insight into dynamic processes. Many quantities calculated from simulations can be directly compared to experimental values, while others provide information not available from experiment. For example, the tilt and methyl angles of chains within a self-assembled monolayer and the amount of hydrogen in a diamond-like carbon (DLC) film are measurable in an experiment. In contrast, the atomic contact force on a single substrate atom, i.e., the force on that atom due to the tip atoms only, and the changes in hybridization of a carbon atom within a DLC film during sliding are not quantities that are currently obtainable from experiments. Herein, the computation of many quantities, including the ones discussed above, and the unique insights that they provided into compression, friction, and wear are discussed.

We investigate the dynamics of physisorbed atomic hydrogen on several carbon based materials (various fullerenes and a graphene sheet) using first principles molecular dynamics simulations. The physisorbed H atoms, generated upon H 2 dissociative chemisorption on metal catalysts and interaction with carbonized "bridge" materials and substrates (Chen, L.; et al. J. Phys. Chem. C 2007, 111, 18995), can diffuse freely on carbon surfaces with high mobility. Our results indicate that physisorption of H atoms is a metastable state and the atoms will readily recombine to form H 2 molecules, which can be recycled to generate more H atoms, or attack the substrates to form C-H bonds. The strength of the resulting C-H bonds exhibits a strong dependency on the carbon material surface curvature. The implication of C-H bond strength on the dehydrogenation of hydrogenated carbon materials to form molecular H 2 is discussed.

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.