Computational Studies of van der Waals Molecules

Paul E. S. Wormer
Institute of Theoretical Chemistry, NSR Center,
University of Nijmegen, Toernooiveld 1,
6525 ED Nijmegen, The Netherlands.

Intermolecular (also known as `van der Waals') forces play an important role in many branches of the physical sciences. Van der Waals molecules are clusters consisting of two or more stable monomers that are held together by van der Waals forces. Because the experimental/theoretical study of Van der Waals molecules gives detailed quantitative information about intermolecular forces, these molecules receive much attention from experimentalists and theoreticians. A pivotal property of a van der Waals molecule is its potential energy surface (PES), which describes the interaction between its monomers as a function of relative coordinates (distances and orientations).

In the present talk three examples will be discussed of theoretical work recently performed in Nijmegen. The first example is the weakly bound van der Waals molecule argon-methane. Its PES was calculated and from this also the rovibrational quantum mechanical motion of its monomers. Energy differences between the rovibrational states are typically a few tens of reciprocal centimeters and were seen experimentally by R. Miller (Chapel Hill, USA) as sidebands of the vibrational methane v₃ transition. The second example is the hydrogen-bonded water dimer. Again, its PES and rovibrational spectrum were calculated. The spectrum is compared with the experimental far-infrared spectrum of R. Saykally and coworkers (Berkeley, USA). The talk will be ended with a few words about ongoing computational studies of the acidic dissociation of HCl in clusters of four and five water molecules.