Energy barrier for carbon nanotube collapse

Small-diameter carbon nanotubes have circular cross section shapes, but the ground state of large diameter tubes correspond to a collapsed structure, stabilized by the van der Waals attraction of opposite sides of the nanotube wall. For those tubes, the circular cross section shape is metastable and it is interesting to investigate the energy barrier for jumping from one configuration to another. Previous theoretical works calculate the energy barrier by considering a transition pathway in which the nanotube collapses uniformly along its length, normally using periodic boundary conditions along the nanotube axis. This assumption is unphysical since it would give an infinite barrier for a nanotube of infinite length. In this work, we calculate the true energy barrier for carbon nanotube collapse by considering a transition pathway that consists of a local deformation that propagates itself along the carbon nanotube axis. This leads to finite and physically meaningful energy barriers in the limit of infinite nanotubes. For typical nanotube diameters, the energy barriers are so large than effectively prevent the collapse induced by temperature of an infinite nanotube. We also perform classical molecular dynamics that confirm these results.