The structural and bonding properties of small neutral alkali halide clusters, (AX)_n with n<10, A=Li^+,Na^+,K^+,Rb^+, and X=F^-,Cl^-,Br^-,I^-, are studied using the ab initio perturbed ion (PI) model and a
restricted structural relaxation criterion. A trend of competition between rocksalt and hexagonal ringlike isomers
is found and discussed in terms of the relative ionic sizes. The main conclusion is that an approximate
value of r_C /r_A=0.5 (where r_C and r_A are the cationic and anionic radii) separates the hexagonal from the
rocksalt structures. The classical electrostatic part of the total energy at the equilibrium geometry is enough to
explain these trends. The magic numbers in the size range studied are n=4, 6, and 9, and these are universal
since they occur for all alkali halides and do not depend on the specific ground-state geometry. Instead those
numbers allow for the formation of compact clusters. Full geometrical relaxations are considered for (LiF)_n
(n=3 – 7) and (AX)_3 clusters, and the effect of Coulomb correlation is studied in a few selected cases. These
two effects preserve the general conclusions achieved thus far.
