What do you mean with "number of atom increases"? In solid iron, the atoms are always in a lattice. The lattice structure can depend on pressure and temperature (and the presence of other atoms), you can look it up in phase diagrams.
thanks for the reply, but my question has not been answered .am trying to use quantum espresso to determine surface energy of iron.to do this, i need to increase the number of slab or layers. i need to know how to increase the atomic position, when increasing the layers.Please,i need someone to help me out