Optimized structures and bonding energies have been calculated for ammonia clusters from n=3 to n=18 using a pairwise additive model potential. The trimer and tetramer are stable cyclic configurations. From the pentamer onward the structures are three dimensional with an increasing tendency to amorphous behavior. The exceptions are the heptamer with a C-s axis, the hexadecamer with a central atom, and the very stable and completely symmetric dodecamer with the D-6h point group. Here each ammonia molecule is bound by two covalent and two hydrogen bonds. In general, the coordination number increases from 2.0 for the rings over 4.0 for n=12 to 4.2 for n=18. The structures agree where available with previously obtained results for a more elaborate potential.