Flux-side correlation function ($C_{fs}(t)$)

A time correlation $C_{fs}(t)=⟨\delta (s_0){\dot{s}}_0\Theta (s_t)⟩$ correlating signed initial flux through a dividing surface with product-side occupation at a later time. Dividing by reactant population gives a rate on a plateau after microscopic recrossing and before macroscopic equilibration.