  Atomistic molecular dynamics simulations have reached a degree of maturity
that makes it possible to investigate the lipid polymorphism of model bilayers
over a wide range of temperatures. However if both the fluid $L_{\alpha}$ and
tilted gel $L_{\beta'}$ states are routinely obtained, the $P_{\beta'}$ ripple
phase of phosphatidylcholine lipid bilayers is still unsatifactorily described.
Performing simulations of lipid bilayers made of different numbers of DPPC
(1,2-dipalmitoylphosphatidylcholine) molecules ranging from 32 to 512, we
demonstrate that the tilted gel phase $L_{\beta'}$ expected below the
pre-transition cannot be obtained for large systems ($>$ 94 DPPC molecules)
through common simulations settings or temperature treatments. Large systems
are instead found in a disordered gel phase which display configurations,
topography and energies reminiscent from the ripple phase $P_{\beta'}$ observed
between the pretransition and the main melting transition. We show how the
state of the bilayers below the pretransition can be controlled and depends on
thermal history and conditions of preparations. A mechanism for the observed
topographic instability is suggested.
