Variation Principle for Calculation of Many-Particle Effects in Crystals
  Variation principle has been developed to calculate many-particle effects in crystals. Within the framework of quasi-particle concept the variation principle has been used to find one-electron states with taking into account of effects due to non-locality of electronic density functional in electromagnetic fields. A secondary quantized density matrix was used to find the Green function of a quasiparticle and changes of its effective mass due to correlated motion of interacting electrons. 
Halina V. Grushevskaya and Leonid I. Gurskii