A string reaction coordinate for the folding of a polymer chain

TL;DR

This paper develops a non-linear string reaction coordinate combining energy and crystallinity to better understand the polymer chain's coil-to-crystal transition, revealing stage-specific controlling factors.

Contribution

It introduces a novel non-linear string reaction coordinate that captures different stages of polymer crystallization more accurately than linear models.

Findings

The transition is first-order like from coil to crystalline state.

The reaction coordinate combining energy and Q6 is most effective.

Different stages are controlled by different degrees of freedom.

Abstract

We investigate the crystallization mechanism of a single, flexible homopolymer chain with short range attractions. For a sufficiently narrow attractive well, the system undergoes a first-order like freezing transition from an expanded disordered coil to a compact crystalline state. Based on a maximum likelihood analysis of committor values computed for configurations obtained by Wang-Landau sampling, we construct a non-linear string reaction coordinate for the coil-to-crystal transition. In contrast to a linear reaction coordinate, the string reaction coordinate captures the effect of different degrees of freedom controlling different stages of the transition. Our analysis indicates that a combination of the energy and the global crystallinity parameter Q6 provide the most accurate measure for the progress of the transition. While the crystallinity parameter Q6 is most relevant in the initial stages of the crystallization, the later stages are dominated by a decrease in the potential energy.