  X-ray grating spectra provide the confirmation of continued mass loss from
novae in the super-soft source (SSS) phase of the outburst. In this work
expanding nova atmosphere models are developed and used to study the effect of
mass loss on the SSS spectra. The very high temperatures combined with high
expansion velocities and large radial extension make nova in the SSS phase very
interesting but also difficult objects to model.
  The radiation transport code PHOENIX was applied to SSS novae before, but
careful analysis of the old results has revealed a number of problems which
lead to new methods and improvements to the code: 1) an improved NLTE module (a
new opacity formalism, rate matrix solver, global iteration scheme, and
temperature correction method); 2) a new hybrid hydrostatic-dynamic nova
atmosphere setup; 3) the models are treated in pure NLTE (no LTE approximation
for any opacity).
  With the new framework a modest amount of models (limited by computation
time) are calculated. These show: 1) systematic behaviour for various
atmospheric conditions, 2) the effect of expansion on the model spectrum is
significant, and 3) the spectra are sensitive to the details of the atmospheric
structure. The models are compared to the ten well-exposed grating spectra
presently available: 5x V4743 Sgr, 3x RS Oph, and 2x V2491 Cyg. Although the
models are on a coarse grid they do match the observations surprisingly well.
  Also, hydrostatic models are computed. The reproduction of the data is
clearly inferior to the expanding models and, more importantly, their
interpretation with hydrostatic models leads to conclusions opposite to those
from expanding models.
  The models enable the derivation of accurate constraints on the physical
conditions deep in the nova atmosphere that are revealed only in the SSS phase.
