How do we add vacuum in a 1D or 2D system simulated in a SIESTA calculation? How much should be the length of this vacuum?
Vacuum is where no atoms exist in the unit-cell. Creating a larger unit-cell in the desired direction is equivalent to vacuum.
For example, to add vacuum in graphene monolayer, you should increase the lattice parameter of the graphene unit cell in the out-of-plane direction to enlarge the vacuum thickness in between neighboring monolayers of graphene.
The vacuum should be chosen large enough to avoid spurious interactions between periodic images. You should try to increase the added vacuum until you see no effect (within some tolerance indeed) on the quantities of your interest.
Notably, in SIESTA vacuum is not “expensive” (as it is in a plane-wave code).
A SIESTA tutorial about converging the size of the box can be found at http://personales.unican.es/junqueraj/JavierJunquera_files/Metodos/Convergence/Convergence.html,
point 3 “Convergence of the structural properties of a dipolar molecule with respect to the size of the supercell”.