  A theoretical approach was developed for an exact numerical description of a
pair of ultracold atoms interacting via a central potential that are trapped in
a three-dimensional optical lattice. The coupling of center-of-mass and
relative-motion coordinates is explicitly considered using a
configuration-interaction (exact-diagonalization) technique. Deviations from
the harmonic approximation are discussed for several heteronuclear alkali-metal
atom pairs trapped in a single site of an optical lattice. The consequences are
discussed for the analysis of a recent experiment [C. Ospelkaus et al, Phys.
Rev. Lett. 97, 120402 (2006)] in which radio-frequency association was used to
create diatomic molecules from a fermionic and a bosonic atom and to measure
their binding energies close to a magnetic Feshbach resonance.
