Multi-Instanton Effects in QCD Sum Rules for the Pion
  Multi-instanton contributions to QCD sum rules for the pion are investigated within a framework which models the QCD vacuum as an instanton liquid. It is shown that in singular gauge the sum of planar diagrams in leading order of the $1/N_{c}$ expansion provides similar results as the effective single-instanton contribution. These effects are also analysed in regular gauge. Our findings confirm that at large distances the correlator functions are more adequately described in the singular gauge rather than in the regular one. 
A.E. Dorokhov, S.V. Esaibegyan, N.I. Kochelev and N.G. Stefanis