  Within non-equilibrium Green's function technique on the real-time contour
and the two-particle-irreducable (2PI) $\Phi$-functional method, a
non-equilibrium potential is introduced. It naturally generalizes the
conventional thermodynamic potential with which it coincides in thermal
equilibrium. Variations of the non-equilibrium potential over respective
parameters result in the same quantities as those of the thermodynamic
potential but in arbitrary non-equilibrium. In particular, for slightly
non-equilibrium inhomogeneous configurations a variation of the non-equilibrium
potential over volume is associated with the trace of the non-equilibrium
stress tensor. The latter is related to the bulk viscosity. This provides a
novel way for evaluation of the bulk viscosity.
