Quantum Monte Carlo calculation of sp-bonded carbon chains: carbynes and carbon rings

Both density functional theory (DFT) and quantum Monte Carlo (QMC) methods have been employed to study ground state properties of two forms of sp-bonded carbon chains: carbynes and carbon rings. According to the DFT-PBE calculations, polyyne, a carbyne structure with alternating single and triple C-C bonds, is energetically favored by 14 meV/atom over cumulene only with double bonds. However, our QMC calculations predict an energy difference of 88(4) meV/atom between them, clearly demonstrating that polyyne is the ground-state structure of carbyne. In the study of carbon rings consisting of 4n and (4n+2) C atoms, DFT calculations show electrons to be delocalized even in the regime where dimerization effects should be large. In contrast, for n>4, QMC calculations indicate the greater stability of a Peierls insulator configuration over a ground state predicted by Huckel’s rule. This qualitative difference between QMC and DFT results shows that many-body correlation effects taken into account by the QMC method are necessary to accurately describe even simple carbon systems such as sp-bonded carbon chains.