Translation invariant calculation of nonperturbative quarkonium mass shifts

The mass shifts of $\text{q}\text{q}$ bound states in the nonperturbative QCD vacuum are calculated by applying field theoretic perturbation theory as derived within the formalism of the Bethe-Salpeter equation. We obtain corrections to heavy quarkonium masses of the order of, but somewhat smaller than those of previous authors, who employed ordinary perturbation theory of nonrelativistic quantum mechanics.