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The atomistic origin of fatigue failure in micron-sized silicon devices is not fully understood. Two series of density-functional theory calculations on cubic diamond Si explore the effect of surface bond formation on crack healing in systems which exhibit strong surface reconstruction. Both series introduce a separation between Si(100) layers (i.e., the crack) and allow the ions to relax to their minimum-energy configuration. The initial surface ionic positions are either bulk terminated or 2×1 reconstructed. A plot of the energy versus the introduced separation reveals that once the surfaces reconstruct, the crack is no longer able to return to the equilibrium configuration. Rather, the healed crack interface contains defects which places the flawed energy minimum at a finite strain of 3% and an increased energy of 1.13 J/m2 relative to the equilibrium configuration. The irreversible plastic deformation supports the mechanism proposed by Kahn et al. [Science 298 1215 (2002)] that invokes mechanically induced subcritical cracking to explain the delayed onset of failure. [ABSTRACT FROM AUTHOR]

The studies were conducted to understand the dynamic interactions between unbound 1,3-cyclohexadiene (CHD) with surface Si dimers and surface CHD adducts.

It has been possible to produce complete active space self-consistent field ab initio molecular dynamics simulations of the preparation of the metastable species vinylidene. There is evidence of the breakdown of transition-state theory because of extensive CD2 rock-local CCD bend mode coupling.

The first-principles prediction of dislocation nucleation in metallic systems subject to realistically sized indenters requires a multiscale approach due to the prohibitive computational expense. The largest empirical atomistic simulations include at most a billion atoms, at the same time requiring the parameterization of new interactions whenever an additional species or crystal structure is added. The multiscale orbital-free density functional theory–local quasicontinuum (OFDFT-LQC) method overcomes these problems by using first-principles OFDFT to capture the atomic interactions while relying upon LQC to evolve the macroscopic system. We use this method to indent the (111) surface of a 2×2×1μm piece of L1 2   Al 3 Mg. Using a localization criterion, the first dislocation is predicted to form off-axis on the slip plane in the direction after the indenter has penetrated 70  nm. Other popular dislocation nucleation criteria give different predictions. These results are strikingly similar to those for indentation into the (111) surface of Al, indicating that the underlying crystal structure, not the atomic identity, is the most important factor in determining the onset of plasticity. [ABSTRACT FROM AUTHOR]

Presents a complete active space self-consistent field ab initio molecular dynamics simulation of the preparation of vinylidene. Isomerization of vinylidene to acetylene; Determination of the average lifetime for vibrationally excited vinylideen-d[sub 2]; Dynamical motion responsible for the multiple barrier recrossing.