====== エネルギーカットオフ ======
===== set_pw =====
  set_pw(self, pw)
set the planewave cutoff.
 
==== Parameters ====
| pw | integer | the planewave cutoff in eV |
 
this function checks to make sure the density wave cutoff is
greater than or equal to the planewave cutoff.

===== set_dw =====
  set_dw(self, dw)
set the density wave cutoff energy.
 
==== Parameters ====
|dw | integer| the density wave cutoff |
 
The function checks to make sure it is not less than the
planewave cutoff.
 
Density_WaveCutoff, $E_{cut}$, describes the kinetic energy necessary to
represent a wavefunction associated with the total density,
i.e. $\vec{G}$-vectors for which $|\vec{G}|^2<4E_{cut}$ will be used to describe the total
density (including augmentation charge and partial core
density). If Density_WaveCutoff is equal to PlaneWaveCutoff
this implies that the total density is as soft as the
wavefunctions described by the kinetic energy cutoff
PlaneWaveCutoff. If a value of Density_WaveCutoff is specified
(must be larger than or equal to PlaneWaveCutoff) the program
will run using two grids, one for representing the
wavefunction density (softgrid_dim) and one representing the
total density (hardgrid_dim). If the density can be
reprensented on the same grid as the wavefunction density
Density_WaveCutoff can be chosen equal to PlaneWaveCutoff
(default).

===== get_pw =====
  get_pw(self)
return the planewave cutoff used
  print jacapo.get_pw()

===== get_dw =====
  get_dw(self)
return the density wave cutoff