  Sum rules for the nonperturbative piece of correlators (specifically, the
vector current correlator) are discussed. The sum rule subtracting the
perturbative part is of the superconvergent type. Thus it is dominated by the
bound states and low energy production cross section. It leads to a
determination of the gluon condensate of $<\alpha_s G^2> = 0.048 \pm 0.039
GeV^4$
