Periapsis Studios

I'm trying to figure out how to procedurally model bismuth crystals, but I'm kind of a noob at Houdini. Any ideas on how I might do this?

For reference, I HAVE determined the rules of hopper crystal growth, which are as follows:

-layers grow on the exposed edges of the existing crystal
-layers do not grow on the "inner surface" of the hopper
-layers can be different thicknesses, but always grow up and out at a 45-degree angle
-outer edges never intersect each other perfectly
-all edges that do not form a closed loop end in an inward turn
-no convex corner edges are exposed
-the inner termination points of spiral structures either extend inwards until they contact another surface or curve inwards to form a spiral on a different plane
-all spirals turn inwards
-incomplete edges can form on inside layers
-non-rectangular closed edges can exist (but all edges must still be at right angles)

I'm trying to figure out the best way to accurately simulate the impact that formed Earth's moon. The idea would be to use particles/grains that each have their own gravitational force. For simplicity, all particles have the same mass and density. My questions are:

1. What type of simulation would be the most optimal for this?

2. How do I give the particles from one planet an initial velocity?

3. How do I set up the simulation so that every particle exerts a gravitational force (weakens by the square of the distance)?