The vacuum state and minimum energy in Dirac's hole theory
  In Dirac's hole theory the vacuum state is assumed to be the state where all negative energy states are occupied and all positive energy states are unoccupied. This is often referred to as the Dirac sea. It is generally assumed that the Dirac sea is the minimum possible energy state. However it will be shown in this paper that this is not the case. 
Dan Solomon
physics.gen-ph quant-ph