  The Mg II h&k lines are amongst the best diagnostic tools of the upper solar
chromosphere. This region of the atmosphere is of particular interest as it is
the lowest region of the Sun's atmosphere where the magnetic field is dominant
in the energetics and dynamics, defining its structure. While highly successful
in the photosphere and lower to mid chromosphere, numerical models have
produced syntheticMg II lines that do not match the observations well. We
present a number of large scale models with magnetic field topologies
representative of the quiet Sun, ephemeral flux regions and plage, and also
models where the numerical resolution is high and where we go beyond the MHD
paradigm. The results of this study show models with a much improved
correspondence with \iris\ observations both in terms of intensities and
widths, especially underscoring the importance of chromospheric mass loading
and of capturing the magnetic field topology and evolution in simulations. This
comes in addition to the importance of capturing the generation of small scale
velocity fields and including non-equilibrium ionization and ion-neutral
interaction effects. Understanding and modeling all these effects and their
relative importance is necessary in order to reproduce observed spectral
features.
