Zero-field-splitting in triplet-state nanotubes

Zero-field-splitting parameters D and E are evaluated for various excited triplet carbon nanotubes. For cylindrical (Cnv) tube geometries only parameter D is nonzero. Parameter E is nonvanishing either for chiral tubes which lack Cnv symmetry, or for those excited states of achiral tubes, which loose Cnv symmetry due to Jahn-Teller distortion. The splitting is found to decrease with increasing tube length and tube diameter. The decay is fast and tends to zero if the excitation affects surface-type states, while D approaches a nonzero limit if the open shell orbitals are of bulk character.