A microscopic theory of photonucleation: Density functional approach for   the properties of a fluid of two-level atoms, a part of which is excited
  We use the density functional method to examine the properties of the nonuniform (two-phase) fluid of two-level atoms, a part of which is excited. Basing on the analysis of the equation of state of a gas of two-level atoms, a part of which is excited, we suggest a simple density functional of the grand thermodynamical potential. We use the proposed density functional of the grand thermodynamical potential to calculate the nucleation barrier for vapor-to-liquid phase transition in the presence of excited atoms. 
Oleg Derzhko and Vasyl Myhal