  Recent lattice model calculations have suggested that a full-layered crystal
surface may undergo, under canonical (particle-conserving) conditions, a
preroughening-driven two-dimensional phase separation into two disordered flat
(DOF) regions, of opposite order parameter. We have carried out extensive
classical molecular dynamics (MD) simulations of the Lennard-Jones fcc(111)
surface, to check whether these predictions are relevant or not for a realistic
continuous system. Very long simulation times, a grid of temperatures from
(2/3)Tm to Tm, and unusually large system sizes are employed to ensure full
equilibrium and good statistics. By examining layer-by-layer occupancies,
height fluctuations, sublattice order parameter and X-ray structure factors, we
find a clear anomaly at ~0.83Tm. The anomaly is distinct from roughening (whose
incipiency is also detected at ~0.94Tm), and is seen to be consistent with the
preroughening plus phase separation scenario.
