and X-rays and increases the recombination rate on dust grains [@Sano+etal2000; @Fromang+etal2002]. [@LesurLongaretti2007; @LesurLongaretti2010] have furthermore shown that the ratio of viscosity to resistivity, the magnetic Prandtl number, affects both small-scale and large-scale dynamics of saturated magnetorotational turbulence. Ideally all these effects should be taken into account. However, in this paper we choose to focus on the dynamics of solid particles in gas turbulence. Thus we include many physical effects that are important for particles (drag forces, self-gravity, collisions), while we ignore many other effects that would determine the occurrence and strength of gas turbulence. This approach allows us to perform numerical experiments which yield insight into planetesimal formation with relatively few free parameters.

![Maxwell and Reynolds stresses as a function of time. The Reynolds stress is approximately five times lower than the Maxwell stress. There is a marked increase in the turbulent stresses when increasing the resolution from $256^3$ to $512^3$ at a fixed mean vertical field $B_0=0.0015$, likely due to better resolution of the most unstable MRI wavelength at higher resolution. Using $B_0=0.003$ at $256^3$ gives turbulence properties more similar to $512^3$.[]{data-label="f:stress_t"}](stress_t.eps){width="\linewidth"}

Initial conditions
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The gas is initialised to have unit density everywhere in the box. The 