We present an ab initio molecular dynamics study of the ground state, finite temperature, dynamical and electronic properties of Na20 and Na10K10 microclusters. For Na20 the lowest-energy structure has several bulklike features. The vibrational spectrum at 200 K is compared to that of the liquid. From our results we derive a rationale for the so far unexplained dependence of the mass spectrum on the nozzle temperature and compositional abundance of the mixed clusters. The implications of our calculations on the validity of the shell model are discussed in detail. © 1989 IOP Publishing Ltd.