  The identification of the interfacial molecules in fluid-fluid equilibrium is
a long-standing problem in the area of simulation. We here propose a new point
of view, making use of concepts taken from the field of computational geometry,
where the definition of the "shape" of a set of point is a well-known problem.
In particular, we employ the $\alpha$-shape construction which, applied to the
positions of the molecules, selects a shape and identifies its boundary points,
which we will take to define our interfacial molecules. A single parameter
needs to be fixed (the "$\alpha$" of the $\alpha$-shape), and several proposals
are examined, all leading to very similar choices. Results of this methodology
are evaluated against previous proposals, and seen to be reasonable.
