text: As mentioned above, often the atoms at a surface would like to be either closer together or further apart. Countering this, the atoms below (substrate) have a fixed in-plane spacing onto which the surface has to register. One way to reduce the total energy is to have extra atoms in the surface, or remove some. This occurs for the gold (111) surface where there is approximately a 5% higher surface density when it has reconstructed. The misregistry with the underlying bulk is accommodated by having partial partial dislocations between the first two layers. The silicon (111) is similar, with a 7x7 reconstruction with both more atoms in the plane and some added atoms (called adatoms) on top.
  Different is the case for anatase (001) surfaces. Here the atoms want to be further apart, so one row "pops out" and sits further from the bulk.