What stable arrangements of hydrogen-bonded water exist in the range of 4-20 water molecules? Water clusters made up of various numbers of water molecules have been described.
Are all of these structures theoretical?
What experimental evidence supports any of these structures?

How does the free energy of a particular water cluster (per number of molecules in the cluster) vary from one thermodynamically stable arrangement to another? Is there a general trend, e.g. larger clusters are more stable?

In response to Martin's initial comment:

I am curious about water clusters in the context of receptor-ligand interactions (for example, in protein receptors, enzymes or synthetic host cavities). 

In cases where the cavities of these macromolecules is small (for example, a small enzyme active site) what kinds of water clusters are likely to fill this void in the absence of ligand? 

Put more broadly, does the structure of the water that is filling the cavity destabilize the receptor in such a way that ligand binding is enhanced?

Considering a hypothetical receptor that is known to bind ligands (whose sizes range between 4 and 20 non-hydrogen atoms) in water, what are likely structures for the encapsulated water cluster that will be displaced by the ligands upon binding?
The Cambridge Cluster Database can give you a start with theoretical binding energies, but only one isomer for each size is shown. 

http://www-wales.ch.cam.ac.uk/~wales/CCD/TIP4P-water.html