  Simulated exoplanet transmission spectra are critical for planning and
interpretation of observations and to explore the sensitivity of spectral
features to atmospheric thermochemical processes. We present a publicly
available generic model grid of planetary transmission spectra, scalable to a
wide range of H$_2$/He dominated atmospheres. The grid is computed using the
1D/2D atmosphere model ATMO for two different chemical scenarios, first
considering local condensation only, secondly considering global condensation
and removal of species from the atmospheric column (rainout). The entire grid
consists of 56,320 model simulations across 22 equilibrium temperatures (400 -
2600 K), four planetary gravities (5 - 50 ms$^{-2}$), five atmospheric
metallicities (1x - 200x), four C/O ratios (0.35 - 1.0), four scattering haze
parameters, four uniform cloud parameters, and two chemical scenarios. We
derive scaling equations which can be used with this grid, for a wide range of
planet-star combinations. We validate this grid by comparing it with other
model transmission spectra available in the literature. We highlight some of
the important findings, such as the rise of SO$_2$ features at 100x solar
metallicity, differences in spectral features at high C/O ratios between two
condensation approaches, the importance of VO features without TiO to constrain
the limb temperature and features of TiO/VO both, to constrain the condensation
processes. Finally, this generic grid can be used to plan future observations
using the HST, VLT, JWST and various other telescopes. The fine variation of
parameters in the grid also allows it to be incorporated in a retrieval
framework, with various machine learning techniques.
