  The de Gennes free energy functional of an infinite smectic-A liquid crystal
at the dual point is shown to be topological and to depend only on the number
of screw dislocations and the anisotropy. This result generalizes the existence
of a Bogomol'nyi bound to an anisotropic system. The role of the boundary of a
finite mesoscopic smectic is to provide a mechanism for the existence of
thermodynamically stable screw dislocations. We obtain a closed expression for
the corresponding free energy and a relation between the applied twist and the
number of screw dislocations.
