units, meaning that we set $c_{\rm
s}=\varOmega=\mu_0=\rho_0=1$. Here $\rho_0$ represents the mid-plane gas density, which in our unstratified simulations is the same as the mean density in the box. The time and velocity units are thus $[t]=\varOmega^{-1}$ and $[v]=c_{\rm s}$. The derived unit of the length is the scale height $H\equiv
c_{\rm s}/\varOmega=[l]$. The magnetic field unit is $[B]=c_{\rm
s}\sqrt{\mu_0\rho_0}$.

Self-gravity
------------

The gravitational attraction between the particles is calculated by first assigning the particle mass density $\rho_{\rm p}$ on the grid, using the Triangular Shaped Cloud scheme described above. Then the gravitational potential $\varPhi$ at the grid points is found by inverting the Poisson equation $$\nabla^2 \varPhi = 4 \pi G \rho_{\rm p}$$ using a Fast Fourier Transform (FFT) method (see online supplement of J07). Finally the self-potential of the particles is interpolated to the positions of the particles and the acceleration added to the particle equation of motion (Eq. \[eq:dvpdt\]). We define the strength of self-gravity through the dimensionless parameter $$\tilde{G} = \frac{4 \pi G}{\varOmega^2/\rho_0} \, ,$$ where $\rho_0$ is the gas density in the mid-plane. This is practical since all simulations are run with $\varOmega=\rho_0=c_{\rm s}=1$. Using $\varSigma_{\rm
g}=\sqrt{2\pi} \rho_0 H$ for the gas column density, we 