  We present the results of planet formation N-body simulations based on a
comprehensive physical model that includes planetary mass growth through mutual
embryo collisions and planetesimal/boulder accretion, viscous disc evolution,
planetary migration and gas accretion onto planetary cores. The main aim of
this study is to determine which set of model parameters leads to the formation
of planetary systems that are similar to the compact low mass multi-planet
systems that have been discovered by radial velocity surveys and the Kepler
mission. We vary the initial disc mass, solids-to-gas ratio and the sizes of
the boulders/planetesimals, and for a restricted volume of the parameter space
we find that compact systems containing terrestrial planets, super-Earths and
Neptune-like bodies arise as natural outcomes of the simulations. Disc models
with low values of the solids-to-gas ratio can only form short-period
super-Earths and Neptunes when small planetesimals/boulders provide the main
source of accretion, since the mobility of these bodies is required to overcome
the local isolation masses for growing embryos. The existence of short-period
super-Earths around low metallicity stars provides strong evidence that small,
mobile bodies (planetesimals, boulders or pebbles) played a central role in the
formation of the observed planets.
