  Ahead of upcoming space missions intending to conduct observations of
low-mass stars in the ultraviolet (UV) spectral region it becomes imperative to
simultaneously conduct atmospheric modeling from the UV to the visible (VIS)
and near-infrared (NIR). Investigations on extended spectral regions will help
to improve the overall understanding of the diversity of spectral lines arising
from very different atmospheric temperature regions. Here we investigate
atmosphere models with a chromosphere and transition region for the M2.5V star
GJ 436, which hosts a close-in Hot Neptune. The atmosphere models are guided by
observed spectral features from the UV to the VIS/NIR originating in the
chromosphere and transition region of GJ 436. High-resolution observations from
the Hubble Space Telescope and Calar Alto high-Resolution search for M dwarfs
with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES)
are used to obtain an appropriate model spectrum for the investigated M dwarf.
We use a large set of atomic species considered in nonlocal thermodynamic
equilibrium conditions within our PHOENIX model computations to approximate the
physics within the low-density atmospheric regions. In order to obtain an
overall match for the nonsimultaneous observations, it is necessary to apply a
linear combination of two model spectra, where one of them better reproduces
the UV lines while the other better represents the lines from the VIS/NIR
range. This is needed to adequately handle different activity states across the
observations.
