The geometric and electronic structures of Si-n, Si-n(+), and AlSin-1 clusters (2less than or equal tonless than or equal to13) have been investigated using the ab initio molecular orbital theory under the density functional theory formalism. The hybrid exchange-correlation energy function (B3LYP) and a standard split-valence basis set with polarization functions [6-31G(d)] were employed for this purpose. Relative stabilities of these clusters have been analyzed based on their binding energies, second difference in energy (Delta E-2) and fragmentation behavior. The equilibrium geometry of the neutral and charged Si-n clusters show similar structural growth. However, significant differences have been observed in the electronic structure leading to their different stability pattern. While for neutral clusters, the Si-10 is magic, the extra stability of the Si-11(+) cluster over the Si-10(+) and Si-12(+) bears evidence for the magic behavior of the Si-11(+) cluster, which is in excellent agreement with the recent experimental observations. Similarly for AlSin-1 clusters, which is isoelectronic with Si-n(+) clusters show extra stability of the AlSi10 cluster suggesting the influence of the electronic structures for different stabilities between neutral and charged clusters. The ground state geometries of the AlSin-1 clusters show that the impurity Al atom prefers to substitute for the Si atom, that has the highest coordination number in the host Si-n cluster. The fragmentation behavior of all these clusters show that while small clusters prefers to evaporate monomer, the larger ones dissociate into two stable clusters of smaller size. (C) 2004 American Institute of Physics.