Static structures of strained carbon chains:DFT-modeling vs classical modeling of the chain with Lennard-Jones potential

TL;DR

This study compares the static structures of strained carbon chains modeled by DFT and Lennard-Jones potentials, revealing a universal bifurcation behavior leading to inhomogeneous bi-structures at critical strain levels.

Contribution

It demonstrates that in strained carbon chains, bifurcation leads to inhomogeneous bi-structures, contrasting Lennard-Jones and DFT models, and explores the universality of this phenomenon.

Findings

Bi-structures appear abruptly at critical strain in DFT models.

Bifurcation is associated with potential regions of small forces.

Electron density near bifurcation point analyzed.

Abstract

We proved earlier that in the strained monoatomic chains with Lennard-Jones potential there can exist an equilibrium static bi-structure, which corresponds to N - 1 equal short interatomic bonds and one long bond with inversion in its center (N is the number of atoms of the chain). In the present work, we investigate with the aid of the density functional theory (DFT modeling) similar structures that can exist in the strained carbon chains. In contrast to the Lennard-Jones model, the bi-structures in this case are inhomogeneous (they have short bonds of different lengths) and appear abruptly when the strain exceeds a certain critical value $\eta_c$ as a result of a hard bifurcation of the equilibrium state (an analogue of the first-order phase transition). Such a bifurcation is associated with two different parts of potential, in which the Lennard-Jones forces can be quite small (this is the vicinity of the potential minimum and the potential tail) and, therefore, the forces acting on the particle between the long and short bonds can be equal. Since practically all interatomic potentials possess such features, the above bifurcation is universal, i.e. it must occur for any monoatomic chain. We have studied in detail the properties of the above structures, in particular, the behavior of their parameters with increasing N. With the help of DFT-modeling, the electron density of the above structures near the bifurcation point is investigated.