  In this paper we study the structure of the ice/vapor interface in the
neighborhood of the triple point for the TIP4P/2005 model. We probe the
fluctuations of the ice/film and film/vapor surfaces that separate the liquid
film from the coexisting bulk phases at basal, primary prismatic and secondary
prismatic planes. The results are interpreted using a coupled sine Gordon plus
Interface Hamiltonian model. At large length-scales, the two bounding surfaces
are correlated and behave as a single complex ice/vapor interface. For small
length, on the contrary, the ice/film and film/vapor surfaces behave very much
like independent ice/water and water/vapor interfaces. The study suggests that
the basal facet of the TIP4P/2005 model is smooth, the prismatic facet is close
to a roughening transition, and the secondary prismatic facet is rough. For the
faceted basal face, our fluctuation analysis allows us to estimate the step
free energy in good agreement with experiment. Our results allow for a
quantitative characterization of the extent to which the adsorbed quasi-liquid
layer behaves as water, and explains experimental observations which reveal
similar activation energies for crystals grown in bulk vapor or bulk water.
