  Motivated by the diversity and complexity of two-dimensional crystals formed
by triangular proteins and protein trimers, we have investigated the structures
and phase behavior of hard-disk trimers. In order to mimic specific binding
interactions, each trimer possesses on `attractive' disk which can interact
with similar disks on other trimers via an attractive square-well potential. At
low density and low temperature, the fluid phase mainly consists of tetramers,
pentamers, or hexamers. Hexamers provide the structural motif for a
high-density, low-temperature periodic solid phase, but we also identify a
metastable periodic structure based on a tetramer motif. At high density there
is a transition between orientationally ordered and disordered solid phases.
The connections between simulated structures and those of 2D protein crystals
-- as seen in electron microscopy -- are briefly discussed.
