  A first-principles approach is introduced to calculate electron field
emission characteristics of nanostructures, based on the nonequilibrium Green
function technique combined with the density functional theory. The method
employs atomic-like basis orbitals with large confinement radii and lithium
anode to describe the electron density in the vacuum between the nanostructure
tip and the anode, so takes the presence of emitted current into account. The
simulation results on a capped single-walled carbon nanotube reproduce the
experimental trend closely, in particular, the current saturation and the
deviation from the Fowler-Nordheim behavior.
