Comment on "Density Functional Simulation of a Breaking Nanowire"

TL;DR

This paper critiques a previous density functional simulation study of a breaking sodium nanowire, arguing that the small unit cell size leads to interference effects that compromise the model's realism for single nanocontacts.

Contribution

It highlights the limitations of small unit cell models in accurately representing single nanocontacts in density functional simulations.

Findings

Interference effects are significant in small unit cell models.

Small unit cells can produce artifacts comparable to force oscillations.

Model realism for single nanocontacts is questionable with limited supercell sizes.

Abstract

In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)] described first principles calculations for a breaking Na nanocontact. Their system consists of a periodic one-dimensional array of supercells, each of which contains 39 Na atoms, originally forming a straight, crystalline wire with a length of 6 atoms. The system is elongated by increasing the length of the unit cell. At each step, the atomic configuration is relaxed to a new local equilibrium, and the tensile force is evaluated from the change of the total energy with elongation. Aside from a discontinuity of the force occuring at the transition from a crytalline to an amorphous configuration during the early stages of elongation, they were unable to identify any simple correlations between the force and the number of electronic modes transmitted through the contact. An important question is whether their model is realistic, i.e., whether it can be compared to experimental results obtained for a single nanocontact between two macroscopic pieces of metal. In this Comment, we demonstrate that with such a small unit cell, the interference effects between neighboring contacts are of the same size as the force oscillations in a single nanocontact.