Continuous versus discrete for interacting carbon nanostructures

Abstract

Intermolecular forces between two interacting nanostructures can be obtained by either summing over all the individual atomic interactions or by using a continuum or continuous approach, where the number of atoms situated at discrete locations is averaged over the surface of each molecule. This paper aims to undertake a limited comparison of the continuum approach, the discrete atom-atom formulation and a hybrid discrete-continuum formulation for a range of molecular interactions involving a carbon nanotube, including interactions with another carbon nanotube and the fullerenes C60, C70 and C80 In the hybrid approach only one of the interacting molecules is discretized and the other is considered to be continuous. The hybrid discrete-continuum formulation would enable non-regular shaped molecules to be described, particularly useful for drug delivery systems which employ carbon nanotubes as carriers. The present investigation is important to obtain a rough estimate of the anticipated percentage errors which may occur between the various approaches in any specific application. Although our investigation is by no means comprehensive, overall we show that typically the interaction energies for these three approaches differ on average by at most 10% and the forces by 5%, with the exception of the C80 fullerene. For the C80 fullerene, while the intermolecular forces and the suction energies are in reasonable overall agreement, the point-wise energies can be significantly different. This may in part be due to differences in modelling the geometry of the C80 fullerene, but also the suction energies involve integrals of the energy, and therefore any errors or discrepancies in the point-wise energy tend to be smoothed out to give reasonable overall agreement for the former quantities.