Ab initio total-energy calculations are used to investigate adatom vac
ancies on the Si(111)-(7x7) surface. In striking contrast to recent ex
perimental estimates, vacancy formation energies are calculated to be
0.9 eV on average, with similar to 0.1-eV variations depending on the
type of adatom. We find that faulted or corner adatoms can be removed
more easily than unfaulted or edge adatoms, respectively. Structural r
elaxations induce large changes in the electronic structure of the sur
face states. Calculation of scanning tunneling microscopy (STM) images
show that the predicted variations should be readily observed in diff
erential STM measurements.