  We consider a model decagonal quasicrystal of composition
Al$_{80.1}$Co$_{19.9}$ -- closely related to actual structures, and using
realistic pair potentials -- on a quasilattice of candidate sites. Its ground
state, according to simulations, is a Hexagon-Boat-Star tiling satisfying
Penrose's matching rules. In this note, we rationalize these results in terms
of the potentials; the Al-Co second-neighbor potential well is crucial.
