  New surveys such as ESA's Euclid mission and NASA's Nancy Grace Roman Space
Telescope are planned to map with unprecedented precision the large-scale
structure of the Universe by measuring the 3D positions of tens of millions of
galaxies. It is necessary to develop theoretically modelled galaxy catalogues
to estimate the expected performance and to optimise the analysis strategy of
these surveys. We populate two pairs of 1 (Gpc/h)$^3$ volume dark-matter-only
simulations from the UNIT project with galaxies using the SAGE semi-analytic
model of galaxy formation, coupled to the photoionisation model GET_EMLINES to
estimate their Halpha emission. These catalogues represent a unique suite that
includes galaxy formation physics and - thanks to the fixed-pair technique used
- an effective volume of ~ (5 Gpc/h)$^3$, which is several times larger than
the Euclid survey. We study the abundance and clustering of those model
star-forming Halpha emission-line galaxies (ELGs). For scales greater than ~5
Mpc/h, we find for the ELGs a scale-independent bias with values in the range b
$\in$ [1,4.5], increasing with redshift over the interval z $\in$ [0.5,2].
Model galaxy properties, including their emission-line fluxes are publicly
available.
