Fig. 6. As Fig. 5, but for single and double 1D “wires” (the supercells employed are much larger than sketched here). The occurrence of imaginary phonon modes in the single wire (a) is emphasised by a red arrow. The double wires in panels (b and c), by stark contrast, are dynamically stable (black arrows). The spillover to marginally “negative” wavenumbers is an artifact of the broadening scheme.