  Over many years, computational simulations based on Density Functional Theory
(DFT) have been used extensively to study many different materials at the
atomic scale. However, its application is restricted by system size, leaving a
number of interesting systems without a high-accuracy quantum description. In
this work, we calculate the electronic and structural properties of a
graphene-metal system significantly larger than in previous plane-wave
calculations with the same accuracy. For this task we use a localised basis set
with the \textsc{Conquest} code, both in their primitive, pseudo-atomic orbital
form, and using a recent multi-site approach. This multi-site scheme allows us
to maintain accuracy while saving computational time and memory requirements,
even in our exemplar complex system of graphene grown on Rh(111) with and
without intercalated atomic oxygen. This system offers a rich scenario that
will serve as a benchmark, demonstrating that highly accurate simulations in
cells with over 3000 atoms are feasible with modest computational resources.
