APE smearing and overlap-Dirac operator are combined to filter QCD vacuum configurations. The results obtained from overlap fermions and improved 5Li cooling are compared, both of them exhibit structures of dilute liquid of instanton. Finally the overlap fermions, improved 5Li cooling and APE smearing are combined to calculate the topological charge and identify the structure of QCD vacuum. The results suggest dilute liquid of instanton dominance of topological charge fluctuations in quenched lattice QCD.
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