Two kinds of complexes of molecules exist: the collisional complexes discussed above, which are short lived. Besides, bound (i.e. relatively stable) complexes of two or more molecules exist, the so-called van der Waals molecules. These exist usually for much longer times than the collisional complexes and, under carefully chosen experimental conditions (low temperature, moderate gas density), their rotovibrational band spectra show "sharp" (or resolvable) lines (Heisenberg uncertainty principle), much like ordinary molecules. If the parent molecules are nonpolar, the same induced dipole mechanisms, which are discussed above, are responsible for the observable spectra of van der Waals molecules.