  We present first results of a recently started lattice QCD investigation of
antiheavy-antiheavy-light-light tetraquark systems including scattering
interpolating operators in correlation functions both at the source and at the
sink. In particular, we discuss the importance of such scattering interpolating
operators for a precise computation of the low-lying energy levels. We focus on
the $\bar b \bar b u d$ four-quark system with quantum numbers $I(J^P) =
0(1^+)$, which has a ground state below the lowest meson-meson threshold. We
carry out a scattering analysis using L\"uscher's method to extrapolate the
binding energy of the corresponding QCD-stable tetraquark to infinite spatial
volume. Our calculation uses clover $u$, $d$ valence quarks and NRQCD $b$
valence quarks on gauge-link ensembles with HISQ sea quarks that were generated
by the MILC collaboration.
