  We analyze the nature of the rotational energy level structure of the OCS-He
and OCS-H_2 complexes with a comparison of exact calculations to several
differentdynamical approximations. We compare with the clamped coordinate
quasiadiabatic approximation that introduces an effective potential for each
asymmetric rotor level, with an effective rotation Hamiltonian constructed from
ground state averages of the inverse of the inertial matrix, and investigate
the usefulness of the Eckart condition to decouple rotations and vibrations of
these weakly bound complexes between linear OCS and 4He or H_2. Comparison with
exact results allows an assessment of the accuracies of the different
approximate methods and indicates which approaches are suitable for larger
clusters of OCS with 4He and with H_2. We find the OCS-H_2 complex is
considerably more rigid than the OCS-4He complex, suggesting that semi-rigid
models are useful for analysis of larger clusters of H_2 with OCS.
