   Description:

This is a simulation of pyramid-shaped objects resting on an immobile surface
(resembling graphene).  Each pyramid is built from spherical particles stacked
like cannon-balls, or stacked fruit.  Ordinarily, the stack does not move 
because the particles at the ground layer are immobilized.  However, 
given an initial (small) perturbation the pyramids collapse in an avalanche.

(In this example, the perturbation is due to shock because we (intentionally)
 did not minimize the system before starting the simulation.  This shock 
 causes an avalanche to occur approximately 5000 timesteps later.)

The particles roll down the pyramid and bounce off the "ground". The bouncing
is due to a repulsive external force which is added artificially.
(See the "run.in" file.)  The simulation looks weird without something 
to bounce off of.  So I added a graphene surface at the bottom as scenery. 
(It does not exert any force on the atoms.)

(Random comment: This could be a fun example to illustrate the Boltzmann
 distribution.  Because there is no damping, in a small region, the particle
 heights should eventually approach the Boltzmann distribution for some 
 suitable temperature.)
