To understand the complex mechanisms of reaction, solvation, and diffusion that determine the chemistry of silica in solution, it is necessary to study first the silicate clusters that participate in these processes. We have investigated, by ab initio density functional methods, all silica-based clusters of the form SIxOy(OH)(z), with a maximum of five silicon atoms and two intramolecular condensations, plus the six-silicon ring and the eight-silicon containing cube. Tn this article (part I), we report our results on the structure, charge distribution, and energy of the simpler clusters: the monomer Si(OH)(4), the dimer Si2O(OH)(6), the linear trimer Si3O2(OH)(8), the ring trimer Si3O3(OH)(6), the linear tetramer Si4O3(OH)(10), the ring tetramer Si4O4(OH)(8), the linear pentamer Si5O4(OH)(12), and the cubic cage Si8O12(OH)(8). We also present density functional results for aluminosilicate clusters: Al(OH)(4)(-), Al2O(OH)(6)(2-), and SiAlO(OH)(6)(-). The results for the more complex clusters are presented in the subsequent article (part II). Our studies reveal a wide diversity of structures and consequently of charge distributions and energies for these clusters. that directly influence their chemical behavior, in particular the interaction with other clusters and with the solvent.