Improved density mixing
=======================

:Who:
    Jens Jørgen

For density mixing in real-space, we use a special metric (described
:ref:`here <densitymix>`) for measuring input to output density
changes with more weight on long wavelength changes.  We will try to
do the density mixing in reciprocal space, where the metric is easier
to express and a wave length dependent preconditioning can also easily
be applied.
