This example shows how to use pw.x to calculate propeties of isolated
systems decoupling periodic images by using Martyna-Tuckerman approach
with truncated coulomb interaction.

Three simple systems are considered:
1) a N atom. 
2) a NH4+ ion.
3) a water molecule.

The calculations are performed in a SC cell of dimension 16 bohr It
is possible to explore convergence of the results w.r.t. box size by
editing the script and addind/modifying the variable called BOX_SIZE_LIST.
Values for BOX_SIZE_LIST = " 12 16 20 24" are provided in the reference

Relevant variables in pw.x input is assume_isolated in namelist SYSTEM 
