When Is A Semiclassical Approximation Self-consistent?

A general condition for the self-consistency of a semiclassical approximation to a given system is suggested. It is based on the eigenvalue distribution of the relevant Hessian evaluated at the streamline configurations (configurations that almost satisfy the classical equations of motion). The semiclassical approximation is consistent when there exists a gap that separates small and large eigenvalues and the spreading among the small eigenvalues is much smaller than the gap. The idea is illustrated in the case of the double-well potential problem in quantum mechanics. The feasibility of the present idea to test instanton models of QCD vacuum is also briefly discussed.