6D-GIM produces samples of universe-flow of dark matter particles at the virial radius. The flow of dark matter particles is a draw from a log normal gaussian random field which satisfy both the mean properties of the infall (temporal decrease, angular isotropy, preferentially radial kinematics infall with an prefered mean velocity) and their correlations (both in time, angle and velocity) as described in the following paper:
The trick is to generate the positions and crossing time on the one hand, and then for the corresponding hypercube (3 components of the velocity and time) draw velocities at the constraint crossing time. This process insures temporal coherence of the draw both in angle and velocity space.
One needs to specify
the total number of sought particles
optionally the parameters characterizing the infall;
The file format is a gadget snapshot with either the two angles and and time encoded as positions, and velocities or the positions and velocities on the unit sphere and the index as a time tag. (i.e. particle 1 is the first particle to enter the virial radius, particle n is the last particle to enter).
it may return the density maps from which the particles were drawn (for the purpose of gaseous infall or gridbased codes).
CAVEAT 1 At this stage there is no consistency between the kinematic space and the position space. i.e. virialized infall does not generically obey phase space conservation. This is a serious challenge.
CAVEAT 2 alt azimuthal periodicity is strictly imposed. This will require a minor fix moving from cartesian GRF to spherical harmonics GRF.