The potentials used in the description of heavy quarkonium by the Schrödinger equation do not take into account nonlinear effects. A procedure for the calculation of nonlinear contributions in perturbation theory is proposed. This procedure involves the adiabatic (Born-Oppenheimer) approximation of the field theory with heavy fermions interacting with a boson field by the theory of a boson field with a quasistatic source. The ground-state energy of the approximate theory is calculated perturbatively with the Gell-Mann Low formula. It is shown by an explicit calculation that nonlinear contributions in QCD are not negligible (as they are in QED).