  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.
