Intermolecular potentials with flexible monomers

Most available intermolecular potentials assume rigid monomers. Such an ass umption is a reflection of a trivial observation that if monomer degrees of freedom are considered, the dimensionality of an intermolecular potential increases dramatically even for few-atom monomers. This puts strict limits on the sizes of clusters with flexible monomers that can be treated by ab i nitio methods since the number of grid points needed to Dt a potential surf ace quickly becomes enormous. Most of the literature flexible-monomer poten tials were obtained from the so-called site-site rigid-monomer potentials b y simply allowing the sites to move with atomic nuclei as the monomers are deformed. This simple atom-following approach has been investigated in the present work, using water dimer and Ar-HF as test systems, and shown to be not adequate. A method has been proposed which uses properties of isolated flexible monomers to improve upon the atom-following approach without perfo rming any dimer calculations outside the rigid-monomer grid. The method is shown to give for Ar-HF a 3D representation of the dispersion energy that s hould be adequate for HF motion in the ground and first excited vibrational states.