  We present a comparative study of the rotational characteristics of various
molecule-doped 4He clusters using quantum Monte Carlo techniques. The
theoretical conclusions obtained from both zero and finite temperature Monte
Carlo studies confirm the presence of two different dynamical regimes that
correlate with the magnitude of the rotational constant of the molecule, i.e.,
fast or slow rotors. For a slow rotor, the effective rotational constant for
the molecule inside the helium droplet can be determined by a microscopic
two-fluid model in which helium densities computed by path integral Monte Carlo
are used as input, as well as by direct computation of excited energy levels.
For a faster rotor, the conditions for application of the two-fluid model for
dynamical analysis are usually not fulfilled and the direct determination of
excitation energies is then mandatory. Quantitative studies for three molecules
are summarized, showing in each case excellent agreement with experimental
results.
