  Strontium clusters of size 2 to 20 atoms have been studied within the local
density approximation using the ab-initio molecular dynamics method with a
plane wave basis in which the s and p electron wavefunctions are well
represented. The resulting low lying isomers are optimized using a
full-potential linear muffin tin orbital method which treats the d-electrons
also on an equal footing. We find that the d electrons contribute significantly
to the binding energy as the cluster size grows, expedite the onset of
metallicity and induce icosahedral growth. These results are in complete
agreement with the experimental finding of rare-gas like magic clusters of
group IIA elements and suggest that the observation of icosahedral growth in
some transition metal clusters may have a similar origin.
