Thermodynamics of small superconductors with fixed particle number
  The Variation After Projection approach is applied for the first time to the pairing hamiltonian to describe the thermodynamics of small systems with fixed particle number. The minimization of the free energy is made by a direct diagonalization of the entropy. The Variation After Projection applied at finite temperature provides a perfect reproduction of the exact canonical properties of odd or even systems from very low to high temperature. 
Danilo Gambacurta and Denis Lacroix
nucl-th cond-mat.supr-con