Instantons and the QCD Vacuum Wavefunctional
  We analyze the instanton transitions in the framework of the gauge invariant variational calculation in the pure Yang-Mills theory. Instantons are identified with the saddle points in the integration over the gauge group which projects the Gaussian wave functional onto the gauge invariant physical Hilbert space. We show that the dynamical mass present in the best variational state provides an infrared cutoff for the instanton sizes. The instantons of the size $\rho<1/M$ are suppressed and the large size instanton problem arising in the standard WKB calculation is completely avoided in the present variational framework. 
William E. Brown, Juan P. Garrahan, Ian I. Kogan and Alex Kovner