  We study the stress distribution profiles and the height and thickness
fluctuations of lipid membranes in the tilted gel state by Monte Carlo
simulations of a generic coarse-grained model for lipid membranes, which
reproduces many known properties of DPPC bilayers. The results are related to
the corresponding properties of fluid membranes, and to theoretical predictions
for crystalline and hexatic membranes. One striking observation is that the
spontaneous curvature of the monolayers changes sign from the fluid to the gel
phase. In the gel phase, the long-wavelength height fluctuations are
suppressed, and the fluctuation spectrum is highly anisotropic. In the
direction of tilt, it carries the signature of soft modes that are compatible
with the wavelength of the ripple phase, which emerges in the transition region
between the fluid and the gel state. In the direction perpendicular to the
tilt, the thickness fluctuations are almost entirely suppressed, and the height
fluctuations seem to be dominated by an interfacial energy, i.e., by
out-of-layer fluctuations, up to length scales of tens of nanometers.
