the Initial Mass Function of gravitationally bound clumps involves masses ranging from a significant fraction of to several times the mass mass of Ceres. We point out that the physical nature of the collisions is unclear, since our numerical algorithm does not allow clumps to contract below the grid size. Gravitational scattering and binary formation are other possible outcomes of the close encounters, in case of resolved dynamics. Finding the Initial Mass Function of planetesimals forming from the gravitationally bound clumps will ultimately require an improved algorithm for the dynamics and interaction of bound clumps as well as the inclusion of particle shattering and coagulation during the gravitational contraction.

The paper is organised as follows. In [Sect. \[s:equations\]]{} we describe the dynamical equations for gas and particles. [Sect. \[s:code\]]{} contains descriptions of a number of improvements made to the Pencil Code in order to be able to perform particle-mesh simulations at up to at least 4096 cores. In [Sect. \[s:parameters\]]{} we explain the choice of simulation parameters. The evolution of gas turbulence and large-scale pressure bumps is analysed in [Sect. \[s:gas\]]{}. Particle concentration in simulations with no self-gravity is described in [Sect. \[s:particles\]]{}. Simulations including particle self-gravity are presented in 