Carbon nanotube chiral angle ($\theta$)

The angle of the wrapping vector from the zigzag direction of graphene. For chiral indices $0\le m\le n$, $\theta ={\tan }^{-1}[\sqrt{3}m/(2n+m)]$, ranging from zero for zigzag tubes to 30 degrees for armchair tubes. Together with diameter, it determines the discrete wrapping geometry, but does not alone determine the observed colour of a nanotube suspension.