  The sources of theoretical uncertainties in the prediction of the two-neutron
drip line are analyzed in the framework of covariant density functional theory.
We concentrate on single-particle and pairing properties as potential sources
of these uncertainties. The major source of these uncertainties can be traced
back to the differences in the underlying single-particle structure of the
various covariant energy density functionals (CEDF). It is found that the
uncertainties in the description of single-particle energies at the two-neutron
drip line are dominated by those existing already in known nuclei. Only
approximately one third of these uncertainties are due to the uncertainties in
the isovector channel of CEDF's. Thus, improving the CEDF description of
single-particle energies in known nuclei will also reduce the uncertainties in
the prediction of the position of two-neutron drip line. The predictions of
pairing properties in neutron rich nuclei depend on the CEDF. Although pairing
properties affect moderately the position of the two-neutron drip line they
represent only a secondary source for the uncertainties in the definition of
the position of the two-neutron drip line.
