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NASA's Kepler mission discovered $\sim700$ planets in multi-planet systemscontaining 3 or more transiting bodies, many of which are super-Earths andmini-Neptunes in compact configurations. Using $N$-body simulations, we examinethe in situ, final stage assembly of multi-planet systems via the collisionalaccretion of protoplanets. Our initial conditions are constructed using asubset of the Kepler 5-planet systems as templates. Two different prescriptionsfor treating planetary collisions are adopted. The simulations address numerousquestions: do the results depend on the accretion prescription?; do theresulting systems resemble the Kepler systems, and do they reproduce theobserved distribution of planetary multiplicities when synthetically observed?;do collisions lead to significant modification of protoplanet compositions, orto stripping of gaseous envelopes?; do the eccentricity distributions agreewith those inferred for the Kepler planets? We find the accretion prescriptionis unimportant in determining the outcomes. The final planetary systems lookbroadly similar to the Kepler templates adopted, but the observed distributionsof planetary multiplicities or eccentricities are not reproduced, becausescattering does not excite the systems sufficiently. In addition, we find that$\sim 1$% of our final systems contain a co-orbital planet pair in horseshoe ortadpole orbits. Post-processing the collision outcomes suggests they would notsignificantly change the ice fractions of initially ice-rich protoplanets, butsignificant stripping of gaseous envelopes appears likely. Hence, it may bedifficult to reconcile the observation that many low mass Kepler planets haveH/He envelopes with an in situ formation scenario that involves giant impactsafter dispersal of the gas disc.

Formation of compact systems of super-Earths via dynamical instabilities and giant impacts