  Given the forthcoming launch of the James Webb Space Telescope (JWST) which
will allow observ- ing exoplanet atmospheres with unprecedented
signal-over-noise ratio, spectral coverage and spatial resolution, the
uncertainties in the atmosphere modelling used to interpret the data need to be
as- sessed. As the first step, we compare three independent 1D
radiative-convective models: ATMO, Exo-REM and petitCODE. We identify
differences in physical and chemical processes taken into ac- count thanks to a
benchmark protocol we developed. We study the impact of these differences on
the analysis of observable spectra. We show the importance of selecting
carefully relevant molecular linelists to compute the atmospheric opacity.
Indeed, differences between spectra calculated with Hitran and ExoMol exceed
the expected uncertainties of future JWST observations. We also show the
limitation in the precision of the models due to uncertainties on alkali and
molecule lineshape, which induce spectral effects also larger than the expected
JWST uncertainties. We compare two chemical models, Exo-REM and Venot Chemical
Code, which do not lead to significant differences in the emission or
transmission spectra. We discuss the observational consequences of using
equilibrium or out-of- equilibrium chemistry and the major impact of phosphine,
detectable with the JWST.Each of the models has benefited from the benchmarking
activity and has been updated. The protocol developed in this paper and the
online results can constitute a test case for other models.
