Ewald summation on a helix : a route to self-consistent charge density-functional based tight-binding objective molecular dynamics

We explore the generalization to the helical case of the classical Ewald method, the harbinger of all modern self-consistent treatments of waves in crystals, including ab initio electronic structure methods. Ewald-like formulas that do not rely on a unit cell with translational symmetry prove to be numerically tractable and able to provide the crucial component needed for coupling objective molecular dynamics with the self-consistent charge density-functional based tight-binding treatment of the inter-atomic interactions. The robustness of the method in addressing complex hetero-nuclear nano- and bio-systems is demonstrated with illustrative simulations on a helical boron nitride nanotube, a screw dislocated zinc oxide nanowire, and an ideal DNA molecule

Directional-dependent thickness and bending rigidity of phosphorene

The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional-based potential, we directly uncover the flexibility of individual α, β and γ phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of α and γ allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene

Collapsed carbon nanotubes : from nano to mesoscale via density functional theory-based tight-binding objective molecular modeling

Due to the inherent spatial and temporal limitations of atomistic modeling and the lack of efficient mesoscopic models, mesoscale simulation methods for guiding the development of super strong lightweight material systems comprising collapsed carbon nanotubes (CNTs) are currently missing. Here we establish a path for deriving ultra-coarse-grained mesoscopic distinct element method (mDEM) models directly from the quantum mechanical representation of a collapsed CNT. Atomistic calculations based on density functional-based tight-binding (DFTB) extended with Lennard-Jones interactions allow for the identification of the cross-section and elastic constants of an elastic beam idealization of a collapsed CNT. Application of the DFTB quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and angular symmetries exhibited by twisted and bent CNTs. The multiscale modeling chain established here is suitable for deriving ultra-coarse-grained mesoscopic models for a variety of microscopic filaments presenting complex interatomic bondings

Electronic and Structural Response of Materials to Fast Intense Laser Pulses

In this chapter we review theoretical and experimental studies of the subject indicated in the title:the response of materials to ultrafast and ultra-intense laser pulses. Our primary emphasis is on the semiconductors GaAs and Si, with some discussion of the fullerene C60. Near the end there is also a brief discussion of certain molecules. The theoretical simulations employ tight-binding electron-ion dynamics (TED), a technique which is fully described in the text. The experiments employ sophisticated techniques that have been developed during the past 20 years, and which are described in papers cited in the text. Comparison of the simulations and experiments shows good agreement in all important respects. In the case of the semiconductors GaAs and Si, there is a nonthermal phase transition as the intensity is varied at fixed pulse duration. For GaAs, the transition corresponds to excitation of about 10 % of the valence electrons to the conduction band. For Si, the threshold intensity is approximately the same, but about 15 % of the electrons are excited. These results are qualitatively understandable, because Si has tighter bonding and a smaller band gap

Thermal Transport in Single-Walled Carbon Nanotubes Under Pure Bending

International audienceThe carbon nanotubes' resilience to mechanical deformation is a potentially important feature for imparting tunable properties at the nanoscale. Using nonequilibrium molecular dynamics and empirical interatomic potentials, we examine the thermal conductivity variations with bending in the thermal transport regime where both ballistic and diffusive effects coexist. These simulations are enabled by the realistic atomic-scale descriptions of uniformly curved and buckled nanotube morphologies obtained by imposing objective boundary conditions. We uncover a contrasting behavior. At shorter lengths, the phonon propagation is affected significantly by the occurrence of localized structural buckling. As the nanotube length becomes comparable with the phonon mean free path, heat transport becomes insensitive to the buckling deformations. Our result settles the controversy around the differences between the current experimental and molecular-dynamics measurements of the thermal transport in bent nanotubes