  Rovibrational states of the four dimers formed by the light and the heavy
isotopologues of the methane and the water molecules are computed using a
potential energy surface taken from the literature. The general rovibrational
energy-level pattern characteristic to all systems studied is analyzed
employing two models of a dimer: the rigidly rotating complex and the coupled
system of two rigidly rotating monomers. The rigid-rotor model highlights the
presence of rovibrational sequences corresponding to formally negative
rotational excitation energies, which is explained in terms of the
coupled-rotors picture.
