  High-resolution X-ray spectra of O-type stars revealed less wind absorption
than expected from smooth winds with conventional mass-loss rates. Various
solutions have been proposed, including porous winds, optically thick clumps or
an overall reduction of the mass-loss rates. The latter has a strong impact on
the evolution of the star. Our final goal is to analyse high resolution X-ray
spectra of O-type stars with a multi temperature plasma model in order to
determine crucial wind parameters such as the mass loss rate, the CNO
abundances and the X-ray temperature plasma distribution in the wind. In this
context we are developing a modelling tool to calculate synthetic X-ray
spectra. We present, here, the main ingredients and physics necessary for a
such work. Our code uses the AtomDB emissivities to compute the intrinsic
emissivity of the hot plasma as well as the CMFGEN model atmosphere code to
evaluate the opacity of the cool wind. Following the comparison between two
formalisms of stellar wind fragmentation, we introduce, for the first time in
X-rays, the effects of a tenuous inter-clump medium. We then explore the
quantitative impact of different model parameters on the X-ray spectra such as
the position in the wind of the X-ray emitting plasma. For the first time, we
show that the two formalisms of stellar wind fragmentation yield different
results, although the differences for individual lines are small and can
probably not be tested with the current generation of X-ray telescopes. As an
illustration of our method, we compare various synthetic line profiles to the
observed O VIII {\lambda} 18.97{\AA} line in the spectrum of {\zeta} Puppis. We
illustrate how different combinations of parameters can actually lead to the
same morphology of a single line, underlining the need to analyse the whole
spectrum in a consistent way when attempting to constrain the parameters of the
wind.
