We present a classical interatomic force field for liquid SiO 2 which has been parametrized using the forces, stresses and energies extracted from ab initio calculations. We show how inclusion of more electronic effects in a phenomenological way and parametrization at the relevant conditions of pressure and temperature allow the creation of more accurate force fields. We compare the results of simulations with this force field both to experiment and to the results of ab initio molecular dynamics simulations and show how our procedure leads to comparisons which are greatly improved with respect to the most widely used force fields for silica.

We derive an analytic expression for the average difference between the forces on the ions in a Car-Parrinello simulation and the forces obtained at the same ionic positions when the electrons are at their ground state. We show that for common values of the fictitious electron mass, a systematic bias may affect the Car-Parrinello forces in systems where the electron-ion coupling is large. We show that in the limit where the electronic orbitals are rigidly dragged by the ions the difference between the two dynamics amounts to a rescaling of the ionic masses, thereby leaving the thermodynamics intact. We study the examples of crystalline magnesium oxide and crystalline and molten silicon.We find that for crystalline silicon the errors are very small. For crystalline MgO the errors are very large but the dynamics can be quite well corrected within the rigid-ion model. We conclude that it is important to control the * Current address : International School for Advanced Studies, via Beirut 2-4, effect of the electron mass parameter on the quantities extracted from Car-Parrinello simulations.

The temperature dependence of the band gap of semiconducting single-wall carbon nanotubes (SWNTs) is calculated by direct evaluation of electron-phonon couplings within a "frozen-phonon" scheme. An interesting diameter and chirality dependence of Eg(T ) is obtained, including nonmonotonic behavior for certain tubes and distinct "family" behavior. These results are traced to a strong and complex coupling between band-edge states and the lowest-energy optical phonon modes in SWNTs. The Eg(T ) curves are modeled by an analytic function with diameter and chirality dependent parameters; these provide a valuable guide for systematic estimates of Eg(T ) for any given SWNT. Magnitudes of the temperature shifts at 300 K are smaller than 12 meV and should not affect (n, m) assignments based on optical measurements.PACS numbers: 73.22.-f, 63.22.+m, 71.38.-k The temperature dependence of the band gap (E g ) is one of the fundamental signatures of a semiconductor, providing important insights into the nature and strength of electron-phonon (e-p) interactions. The first measurements of E g (T ) date from the dawn of the semiconductor era [1]. Typically, E g (T ) curves show a monotonic decrease with temperature that is non-linear at low T and linear at sufficiently high T [2, 3].Semiconducting carbon nanotubes are relatively novel semiconductor materials [4], with a variety of potential applications. Despite intensive research since their discovery, it has only recently become possible to perform measurements of the optical gap in individual single-wall carbon nanotubes (SWNTs) [5,6,7,8,9,10]. Such measurements, combined with information from vibrational spectroscopy, provide a route to (n, m) assignment of SWNTs [5,8,9]. Understanding E g (T ) for nanotubes is extremely important in this context, since experiments are usually performed at room-temperature and (n, m) assignments are often guided by comparisons between observed optical transition patterns and the corresponding predictions from calculations at T = 0 K. Moreover, the E g (T ) signature could provide extra information for those assignments.Although it has been demonstrated that many-body quasiparticle and excitonic effects are crucial for the correct description of the photoexcited states and for a quantitative understanding of such optical measurements [11], the single-particle gap is still a fundamental quantity because: (i) it is the starting point for more elaborate descriptions and (ii) trends in the single-particle gap are often preserved by such refinements. Therefore, this work is devoted to describing the temperature dependence of the single-particle band gap of semiconducting SWNTs. From the calculated results for 18 different SWNTs, a complex dependence of E g (T ) on chirality and diameter is found, with an unsual non-monotonic behavior for certain classes of tubes. This behavior arises from the differences in sign of the e-p coupling associated to low-energy optical phonons. A model relation describing E g (T ) for any given SWNT, as a function of...

Molecular dynamics simulations are used to study mechanical energy dissipation in carbon nanotube oscillators of lengths of tens of nanometers. The principal source of friction is found to be the ends of the tubes and hence dynamical friction is virtually independent of the overlap area between tubes. As a result of this, tube commensuration does not lead to significantly increased frictional forces. The friction force is found to depend strongly and nonlinearly on the relative velocity of the tubes. It is suggested that a strong velocity dependence and strong contributions from surface edges may be quite general features of friction at the nanoscale.

We used simulations with a classical force field to study the transformation under hydrostatic pressure of isolated single-walled nanotubes (SWNT) from a circular to a collapsed cross section. Small-diameter SWNTs deform continuously under pressure, whereas larger-diameter SWNTs display hysteresis and undergo a first-order-like transformation. The different behavior is due to the changing proportions in the total energy of the wall-curvature energy and the van der Waals attraction between opposite walls of the tube.

We study the structural and electronic properties of isolated single-wall carbon nanotubes (SWNTs) under hydrostatic pressure using a combination of theoretical techniques: Continuum elasticity models, classical molecular dynamics simulations, tight-binding electronic structure methods, and first-principles total energy calculations within the density-functional and pseudopotential frameworks. For pressures below a certain critical pressure c P , the SWNTs' structure remains cylindrical and the Kohn -Sham energy gaps of semiconducting SWNTs have either positive or negative pressure coefficients depending on the value of ( , ) n m , with a distinct "family" (of the same − n m) behavior. The diameter and chirality dependence of the pressure coefficients can be described by a simple analytical expression. At c P , molecular-dynamics simulations predict that isolated SWNTs undergo a pressure-induced symmetry-breaking transformation from a cylindrical shape to a collapsed geometry. This transition is described by a simple elastic model as arising from the competition between the bond-bending and PV terms in the enthalpy. The good agreement between calculated and experimental values of c P provides a strong support to the "collapse" interpretation of the experimental transitions in bundles.

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Tunable Raman spectroscopy is used to measure the optical transition energies Eii of individual single wall carbon nanotubes. Eii is observed to shift down in energy by as much as 50 meV, from -160 to 300 degrees C, in contrast with previous measurements performed on nanotubes in alternate environments, which show upshifts and downshifts in Eii with temperature. We determine that electron-phonon coupling explains our experimental observations of nanotubes suspended in air, neglecting thermal expansion. In contrast, for nanotubes in surfactant or in bundles, thermal expansion of the nanotubes' environment exerts a nonisotropic pressure on the nanotube that dominates over the effect of electron-phonon coupling.