This example illustrates how to use the phonon code to calculate
the dispersions along some the a path of the BZ for two-dimensional materials.
The Gamma-K-M-Gamma path of monolayer Boron Nitride is used as an example.  
The calculation proceeds as follows 

1) make a self-consistent calculation for BN, setting assume_isolated='2D' 
in the SYSTEM namelist to activate the 2D Coulomb cutoff. 
   (input=bn.scf.in, output=bn.scf.out) 

2) make the phonon calculation for a grid of 881 q-points. The 2D Coulomb 
cutoff is automatically activated for a phonon run done on top of a 2D scf run 
(no need to specify any additional flag in the phonon input).

3) interpolate the phonon dispersion using q2r and matdyn. This time, and especially for polar 2D material, one should set :
loto_2d=.true. 
in input so that LO-TO splitting is treated in the correct two-dimensional framework.