Nonrigid large van der Waals molecules

We address the breakdown of the (nearly) rigid molecule description of molecular structure and spectra, which is realized when the amplitude of atomic motion becomes comparable with the average bond length. This situation prevails for van der Waals molecules and clusters at finite temperatures. Molecular nonrigidity was characterized in terms of an "intramolecular melting" Lindemann criterion, which reveals that aromatic molecule (M)-rare gas (A≡Ar, Kr, Xe) van der Waals molecules (clusters) are nonrigid at T≥10 K. A quantification of the concept of molecular nonrigidity in these systems was provided by constant energy molecular dynamics simulations for the smallest cluster of the M·A_n family, i.e., the one-sided (2|0) benzene · Ar₂ molecule, which reveals a hierarchy of isomerization phenomena with increasing temperature.