What happens when my account balance goes negative?

Your account balance is printed each time you login, so you might see your
account balance heading toward zero quickly if you are using large numbers 
of processors per job or running large numbers of jobs.

Your group account balance should only go negative if your group has used 
more computer time than the amount of compute time represented by the compute ndoes
that your group purchased.
  The monthly amount is simply 
  1/12 of the number of hours in a year
  times the number of compute nodes you purchased
The sum of these is all the available hours on all nodes in an average month.

The policy is implemented as follows:

  Groups that have a negative balance:
  *   are allowed to continued to run, but
  *   have a lower number of total nodes that they can compete for
  *   have a lower number of jobs per user and per group than that for
       groups which have a postive account balance.

  The policy is essentially that which was in effect initially on Condo, but was
found not to fully utilize the machine.

  Groups with positive balances are running under the relaxed scheduling policies
approved by the HPC Committee in Nov 2013, which allowed for increase system

  This means that the groups with positive balances are given priority in use of the
system.  If they are not using it, and the system is relatively idle
and other policies for short duration jobs (24 hours or less) are invoked which
allow all groups to use resources when the system is idle.

This is explained more fully in:

Queues and Scheduling Stucture