  This article covers few selected aspects of quantum theory of molecular
rotations and vibrations. Triatomic molecules are the simplest systems, which
show qualitative characteristics of larger polyatomic molecules. On the minimal
example of triatomic molecules we demonstrate several theoretical methods used
to calculate highly accurate rotational-vibrational-electronic energies,
wavefunctions and transition intensities. The reader can view this article as a
tutorial, by no means comprehensive, which still touches some of the unsolved
problems in modern theoretical molecular spectroscopy.
