  We introduce a scheme to design patchy particles so that a given target
crystal is the global free-energy minimum at sufficiently low temperature. A
key feature is a torsional component to the potential that only allows binding
when particles have the correct relative orientations. In all examples studied,
the target crystal structures readily assembled on annealing from a low-density
fluid phase, albeit with the simpler target structures assembling more rapidly.
The most complex example was a clathrate with 46 particles in its primitive
unit cell. We also explored whether the structural information encoded in the
particle interactions could be further reduced. For example, removing the
torsional restrictions led to the assembly of an alternative crystal structure
for the BC8-forming design, but the more complex clathrate design was still
able to assemble because of the greater remaining specificity.
