The interaction of isolated Cu, Pd, and Cs atoms on regular and defect sites of the SiO2 surface has been studied with cluster models. Hartree-Fock, density functional theory, and multireference configuration interaction methods have been used to characterize the bonding of the metal atoms at the following sites: bridging oxygens at the regular surface, =Si-O-Si=, Si singly occupied sp(3) dangling bonds (E＇ centers), =Si-., nonbridging oxygen centers, =Si-O-., and neutral oxygen vacancies, =Si-Si=. The bonding with the nondefective sites of the surface is very weak, <0.2 eV. Sticking of the atoms occurs only at the defect sites with adsorption energies ranging from 1 to 3 eV. Several spectral signatures of the bond at the silica defects have been considered: core level shifts, impurity levels in the band gap, and optical transitions.