  We study the ferroelectricity in a one-dimensional system composed of a
double helix SnIP with absorbing water molecules. Our ab initio calculations
reveal two factors that are critical to the electrical polarization. The first
one is the orientation of polarized water molecules staying in the R2 region of
SnIP. The second one is the displacement of I atom which roots from subtle
interaction with absorbed water molecules. A reasonable scenario of
polarization flipping is proposed in this study. In the scenario, the water
molecule is rolling-up with keeping the magnitude of its electrical dipole and
changing its direction, meanwhile, the displacement of I atoms is also
reversed. Highly tunable polarization can be achieved by applying strain, with
26.5% of polarization enhancement by applying tensile strain, with only 4%
degradation is observed with 4% compressive strain. Finally, the direct band
gap is also found to be correlated with strain.
