Peierls instability in linear carbon chains calculated by quantum Monte Carlo

Linear carbon chains are known to occur in two different structures: the semi-metallic cumulene (characterized by identical double bonds between the carbon atoms) and the semiconducting dimerized polyyne (characterized by the alternation of triple and single bonds). The Peierls instability that triggers the transition from cumulene to polyyne^1,2 is reflected in the instability of the Longitudinal Optical (LO) mode at Γ³ and is influenced by the long-range correlation between electrons, such as the highest optical phonon mode at the high-symmetry point K in graphene.⁴
Since Density Function Theory (DFT) is known to under- or over-estimate the instability according to the different exchange-correlation functionals, to better assess their behavior, we present here unbiased Variational Monte Carlo (VMC) calculations studying the relative stability of the infinite carbon-atom chain in the two conformations (cumulene and polyyne) as a function of the stretching and we study the variation of the LO frequency at Γ.

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