0) What is ICgen ?
ICgen is builds cosmological initial condition for gadget. Constraints can be put on the initial density field. At the moment it is possible to constrain the velocity field and the density field. However if requested it can be enhanced to support more constraints quite easily (once you know the relationship between the gravitational field and the constraint). The memory is nearly fully shared across nodes except for one thing: the matrix of the constraints which holds N^2 doubles where N is the number of contraints.
More details can be found here .