MEAN-FIELD OUT OF CHAOS

The microscopic dynamics of a quantum many-body system is analyzed und er the assumption that numerous complicated intermediate states in exa ct operator equations of motion are of chaotic nature and can be avera ged out. The averaging procedure, preserving all kinematical constrain ts, is defined by the invariance with respect to phase transformations . The situation with no significant collective correlations is conside red in detail. As a result, the mean-field theory (MFT) emerges accumu lating the smooth component of the total random dynamics. The obtained version of the MFT differs from the Hartree-Fock approximation in tak ing into account the average effect of fluctuations. The mean-field re presentation is suggested to be a natural basis for estimating the deg ree of complexity of generic complicated wave functions.