  The method of the real time perturbative calculations of nonequilibrium
averages is generalised to the case of varying chemical potential. Calculations
are performed in the frame of Zubarev's nonequilibrium density matrix approach.
In this approach perturbations of temperature and other thermodynamical
parameters are taken into account explicitly including nonlinear terms. It
differs from the Schwinger-Keldysh approach through the choice of more general
initial conditions for the density matrix.
