(This file contains old material of the respfn_help.htm
 file, that could be reuse later)

(WARNING for the next paragraph : the stationary formulation
 that was present in RESPFN has not (yet) been implemented in abinit).
The generation of the 2DTE can be made by two different methods.
The first method is stationary, which means that the error in the
2DTE is quadratic with respect of the error in the 1WF. The
second method is non-stationary, with the error in the 2DTE being
linear with respect to the error in the 1WF.
In order to build the 2DTE with respect to perturbations
j1 and j2, the stationary method needs the 1WF with respect
to both j1 and j2, while the non-stationary method is able to
generate it using either the 1WF corresponding to j1 OR the
1WF corresponding to j2.
Also, the stationary method uses more central memory and disk space,
and sometimes the nonstationary method is the only one possible.
It is the user's responsability to insure that the central memory
and disk space are sufficient.
Only one method exists for the calculation of the 3DTE. The central
memory and disk space requirements are even larger than for the stationary
method for calculating 2DTE, and in order to get the 3DTE with respect to
perturbations j1, j2 and j3, the 1WFs with respect to j1, j2 and j3
are needed.
(END of WARNING)

(NOT PRESENT IN VERSION 2.0)
In the second part, the 2DTE and 3DTE are generated, according to
the parameters rfmeth and rf3.
* stationary expression : at some time, all 1RHO
    have to reside in the central memory together, and at some
    other time, all 1WF for one k point also have to be
    in the central memory.
    They are all read from their wf1out(ipert) and
    v1out(ipert) files. Because the memory requirements are large,
    the non-stationary method, albeit less accurate, can be useful.
    The stationary 2DTE is computed in the stady3.f subroutine,
    and the dimensioning of some arrays is to be done directly
    in stady3.f (see the section on memory management).
    The output of the stationary calculation is the set of all
    second derivatives of the total energy that come from the
    mixing of the perturbations that are asked by the user
    in the input file.
* 3DTE : All the 1WF, 1VR and 1RHO have also to reside in
    the central memory. The calculation of 3DTE is in the
    current version of the RESPFN code also
    done inside the stady3.f subroutine.
    (Note : in the present version, the calculation of 3DTE
     has been disabled.)
(END OF NOT PRESENT IN VERSION 2.0)