The full-dimensional quantum-mechanical treatment is the most accurate way to describe anharmonic motions of nuclei. The full solution of the vibrational Schrödinger equation is a computationally and theoretically challenging task. First, it requires extensive quantum-chemical calculations to produce the numerical potential energy surface (PES). Often these data are approximated with a suitable analytical form. Finally, it requires advanced techniques for solving multidimensional Schrödinger equations. The difficulties associated with these procedures limit the widespread usage of such methods.