Elastic constants of sodium from molecular dynamics

Finite Temperature Properties of NiTi from First Principles Simulations: Structure, Mechanics, and Thermodynamics

We present a procedure to determine temperature-dependent thermodynamic properties of crystalline materials from density functional theory molecular dynamics (DFT-MD). Finite temperature properties (structural, thermal, and mechanical properties) of the phases (ground state monoclinic B33, martensitic B19', and austenitic B2) of the shape memory alloy NiTi are investigated. Fluctuation formulas and numerical derivatives are used to evaluate mechanical and thermal properties. A modified version of thermodynamic upsampling is used to enable simulations with lower DFT convergence thresholds. In addition, a thermodynamic integration expression is developed to compute free energies from isobaric DFT-MD simulations that accounts for volume changes. Structural parameters, elastic constants, volume expansion, and specific heats as a function of temperature are evaluated. Phase transitions between B2 and B19' and between B19' and B33 are characterized according to their thermal energy, entropy, and free energy differences as well as their latent heats. Anharmonic effects are shown to play a large role in both stabilizing the austenite B2 phase as well as suppressing the martensitic phase transition. The quasiharmonic approximation to the free energy results in large errors in estimating the martensitic transition temperature by neglecting these large anharmonic components.

Thermodynamic and Elastic Properties of Polyethylene at Elevated Temperatures

Over the years molecular modeling techniques, such as Molecular Mechanics, Monte Carlo and Molecular Dynamics have been applied to study the equilibrium thermodynamic and mechanical properties of materials. The accuracy of the predictions made by these techniques strongly depend on the force fields employed to represent the interactions in the studied system. Recently developed force field parameters for crystalline polyethylene are shown to reproduce the mechanical properties of polyethylene accurately through molecular mechanics. Here, we will present the statistical fluctuation formulae for the elevated temperature equilibrium thermodynamic and elastic properties in terms of microscopic variables such as energy, enthalpy, pressure, volume, microscopic strain or stress tensors and present preliminary results of our calculations.

Thermo-mechanical stability and strength of peptide nanostructures from molecular dynamics: self-assembled cyclic peptide nanotubes

Peptide nanostructures present a wide range of opportunities for applications in biomedicine and bionanotechnology; hence experimental and theoretical studies aiming at determination of thermo-mechanical stability of peptide-based nanostructures are critical for the design and development of their technological applications. Here, we present a homogeneous deformation method combined with the finite elasticity theory and molecular dynamics simulations (MD) for the calculation of second-order anisotropic elastic constants for a membrane model made up of self-assembled cyclic peptide nanotubes. We have computed the values of all anisotropic elastic constants at 300 K. The value of the engineering Young's modulus (in the z direction) is 19.6 GPa. We observed a yield behavior in the z direction for a strain value of 6%. Furthermore, we also report calculated heat capacity, thermal expansion coefficient and isothermal compressibility of the system under study.