  We investigate the ability of a nonlinear force-free code to calculate
highly-twisted magnetic field configurations using the Titov and D\'{e}moulin
(1999) equilibrium field as a test case. The code calculates a force-free field
using boundary conditions on the normal component of the field in the lower
boundary, and the normal component of the current density over one polarity of
the field in the lower boundary. The code can also use the current density over
both polarities of the field in the lower boundary as a boundary condition. We
investigate the accuracy of the reconstructions with increasing flux-rope
surface twist number $N_{\textrm{t}}$, achieved by decreasing the sub-surface
line current in the model. We find that the code can approximately reconstruct
the Titov-D\'{e}moulin field for surface twist numbers up to $N_{\textrm{t}}
\approx 8.8$. This includes configurations with bald patches. We investigate
the ability to recover bald patches, and more generally identify the
limitations of our method for highly-twisted fields. The results have
implications for our ability to reconstruct coronal magnetic fields from
observational data.
