The mechanical behavior of $\ensuremath{\langle}111\ensuremath{\rangle}$ ultrathin gold nanowires (NWs) and their dependence on the correlated parameters of size (i.e., diameter $\ensuremath{\sim}1--1.6$ nm), morphology (rectangular and hexagonal prism), and ultrahigh density twin boundaries was investigated using density functional theory calculations. The parameters of size, morphology, and the presence of twins all significantly influence the mechanical behavior of Au NWs, and their effects are interdependent. Importantly, an ultrahigh density of twins in NWs enhances their strength in both morphologies, and across all sizes studied. Our calculations reveal a remarkable range of Young's modulus (60--158 GPa) and ultimate tensile strengths (3.7--14.0 GPa), suggesting that the scatter observed in experimental research involving ultrathin gold nanowires is a feature of intrinsic complexity. In particular, a ``wrinkling'' of the atomic planes along the wire axis due to a combination of anisotropic surface stresses and wire core response, is demonstrated to be largely suppressed by the presence of high-density twins. The high tensile strengths of the studied nanowires with twins is attributed to a combination of the suppression of this anisotropic phenomena and slip plane disruption. This work demonstrates that precise control of these parameters is required during synthesis to achieve target mechanical behaviour in industrial device applications. Moreover, the work demonstrates that this system has the potential to be tuned for access to a vast range of materials design space.
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