[en] Nonperturbative large scale fluctuation of gluonic field which are present in the exact vacuum of QCD result in modification of interaction between quarks in quarkonium. Such effects are considered for heavy quarks in cases when the size of the quarkonium system is much smaller than that of the dominating gluonic fluctuations. The Green function of relative motion in heavy quarkonium is calculated for energy below threshold with the account of leading nonperturbative effects. It is shown that once nonperturbative gluonic degrees of freedom are taken into consideration they can never be absorbed into a sort of interaction potential between quarks