  The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in
extinction against the diffuse infrared emission of the Galactic plane and in
emission at 850 microns. We use a novel computational technique based on an
advanced genetic algorithm to explore thoroughly 3 different models of
self-gravitating, pressure truncated filaments and to constrain their
parameters. Specifically, the models tested are the non-magnetic Ostriker
(1964) model, a generalized version of the magnetic Stodolkiewicz (1963) model,
and the magnetic Fiege & Pudritz (2000) model. Previous results showed that
G11.11-0.12 has a much steeper r^{-4} radial density profile than other
filaments, where the density varies approximately as r^{-2}, and that this
steep density profile is consistent with the Ostriker (1964) model. We present
a more complete analysis that shows that the radial structure of G11.11-0.12 is
consistent with regimes of each of these models. All of the magnetic models
that agree with the data are threaded by a dominant poloidal magnetic field,
and most have dynamically significant fields. Thus, G11.11-0.12 is an excellent
candidate for radial support by a magnetic field that is predominantly
poloidal. We predict the polarization patterns expected for both magnetic
models and show that the two magnetic models produce different polarization
patterns that should be distingished by observations.
