GRAFIC 1 outputs the files via the unformatted Fortran file format. In these files, the seven fields (ic_deltab, ic_velbx/y/z, ic_velcx/y/z) are described on a regular grid.

The seven outputs follows the same structure:

• A header: a single reccord which contains
  • np1,np2,np3 : 3 integers(kind=4) describing the size of the grid along the 3 directions.
  • dx : the size of a grid element in Mpc/h (real*4)
  • x1o,x2o,x3o: the coordinates of the grid’s origin (real*4).
  • astart: the initial scale factor (real*4)
  • omegam, omegav: and (real*4).
  • h0 : the hubble constant (real*4)

• The ICs data: it consists in np3 reccords of np1*np2 matrix, i.e. ICs are organised in layers along the z-direction. This feature can be easily used to spare memory usage when these files are manipulated, since layers can be uploaded sequentially in memory. Velocities are in km/s while density is expressed in terms of density contrast.

The following fortran example shows how to read such files:

White Noise files follow the same data structure, with a different header format. The header stands as a single record which contains the following variables:

• np1,np2,np3 : 3 integers(kind=4) describing the size of the grid along the 3 directions.
• iseed : the seed of the random generator, integer (kind=4).

Subsequent layers are organised in the same way as the density and velocities fields.