  Radiative transfer (RT) models are critical in the interpretation of
exoplanetary spectra, in simulating exoplanet climates and when designing the
specifications of future flagship observatories. However, most models differ in
methodologies and input data, which can lead to significantly different
spectra. In this paper, we present the experimental protocol of the MALBEC
(Modeling Atmospheric Lines By the Exoplanet Community) project. MALBEC is an
exoplanet model intercomparison project (exoMIP) that belongs to the CUISINES
(Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet
Studies) framework which aims to provide the exoplanet community with a large
and diverse set of comparison and validation of models. The proposed protocol
tests include a large set of initial participating RT models, a broad range of
atmospheres (from Hot Jupiters to temperate terrestrials) and several
observation geometries, which would allow us to quantify and compare the
differences between different RT models used by the exoplanetary community. Two
types of tests are proposed: transit spectroscopy and direct imaging modeling,
with results from the proposed tests to be published in dedicated follow-up
papers. To encourage the community to join this comparison effort and as an
example, we present simulation results for one specific transit case (GJ-1214
b), in which we find notable differences in how the various codes handle the
discretization of the atmospheres (e.g., sub-layering), the treatment of
molecular opacities (e.g., correlated-k, line-by-line) and the default
spectroscopic repositories generally used by each model (e.g., HITRAN, HITEMP,
ExoMol).
