  We calculated the image potentials of single-walled carbon nanotubes of
various structures in the condition of field emission with a quantum chemistry
method. The image potential of the single-walled carbon nanotubes can be well
fitted with the image potential of an ideal metal sphere in a size comparable
to an atom. The image potential is not sensitive to both the applied field and
the structure of the tube. When the image potential is included, the emission
current would be one order larger.
