The orientation of a ${\mathrm{C}}_{70}$ fullerene molecule encapsulated in a single-walled carbon nanotube (SWCNT) depends on the tube radius. First we confirm that chirality effects do not affect the orientation as well by comparing discrete atomistic calculations with the results of a continuous tube approximation for a variety of SWCNTs. The molecular and the tube symmetry are exploited by using symmetry-adapted rotator functions. We accurately determine the optimal molecular orientation as a function of the tube radius; for low $(\lesssim 7\mathrm{\AA })$ and high $(\gtrsim 7.2\mathrm{\AA })$ tube radii, lying and standing molecular orientations are recovered, respectively. In between, we observe a transition regime. In addition, we consider off-axis positions. We perform a one-dimensional liquid description of a chain of on-axis ${\mathrm{C}}_{70}$ molecules inside a SWCNT. All results agree well with recent x-ray diffraction experiments.

• Received 12 September 2006

DOI:https://doi.org/10.1103/PhysRevB.75.045419