  The NPDGamma experiment measures the asymmetry in $\gamma$-ray emission in
the capture of polarized neutrons on liquid parahydrogen. The beam polarization
is measured using $^3$He spin analysis, but this measurement does not account
for the contribution of depolarization from spin-flip scattering primarily due
to orthohydrogen in the bulk liquid. This is a systematic effect that dilutes
the experimental asymmetry and is modeled using Monte Carlo. Methods for
tracking neutron spin in MCNPX were developed in order to calculate the average
neutron polarization upon capture for use as a multiplicative correction to the
measured beam polarization for the NPDGamma experiment.
