Gluon condensate in charmonium sum rules for the axial-vector current
  The charmonium sum rules for the axial-vector current are considered. The three-loop perturbative corrections, operators up to 8 dimension and perturbative corrections to the lowest dimension operator are accounted. The contribution of the operators of 6 and 8 dimensions is computed in the framework of factorization hypothesis and instanton model. For the value of gluon condensate the following result is obtained: (0.005+0.001-0.004)GeV^4 (for the charm quark mass 1.275 GeV). 
A. Samsonov