  We study properties of two different atoms at a single optical lattice site
at a heteronuclear atomic Feshbach resonance. We calculate the energy spectrum,
the efficiency of rf association and the lifetime as a function of magnetic
field and compare the results with the experimental data obtained for K-40 and
Rb-87 [C. Ospelkaus et al., Phys. Rev. Lett. 97, 120402 (2006)]. We treat the
interaction in terms of a regularized delta function pseudopotential and
consider the general case of particles with different trap frequencies, where
the usual approach of separating center-of-mass and relative motion fails. We
develop an exact diagonalization approach to the coupling between
center-of-mass and relative motion and numerically determine the spectrum of
the system. At the same time, our approach allows us to treat the anharmonicity
of the lattice potential exactly. Within the pseudopotential model, the center
of the Feshbach resonance can be precisely determined from the experimental
data.
