More than a dozen cases of nonrigid van der Waals clusters are present
ed and discussed to demonstrate that cluster nonrigidity is a general
phenomenon in all weakly bound systems. The interplay of structure and
nonrigidity complicates cluster research and mandates a dynamical app
roach to cluster properties in which multiple stable configurations co
exist and interconvert, and large amplitude nuclear motions are the ru
le rather than the exception. Empirical potential energy surface calcu
lations are employed to yield physical insight into the structure and
dynamics of nonrigid clusters, and molecular symmetry group theory is
applied to analyze spectroscopic manifestations of cluster nonrigidity
. Empirical potentials of various forms are successful in predicting m
ost cluster structures, as well as estimating the potential surface ba
rrier heights hindering the interconversion between different local mi
nimum-energy structures. Such calculational approaches also emphasize
the importance of large amplitude motion for one or more of the cluste
r vibrational degrees of freedom. The limits of these empirical calcul
ations are discussed, and recent attempts to derive cluster structure
and properties by ab initio techniques are reviewed. The aromatic/smal
l molecule clusters considered in this presentation display two types
of nonrigidity: local nonrigidity in which large amplitude motion invo
lves the rotation of one of the molecular constituents, and global non
rigidity in which large amplitude motion involves displacement of the
centers of mass of the molecular constituents. The former motion inter
changes equivalent atoms, and the latter motion interchanges cluster c
onformations. The potential surface barriers for these large amplitude
motions lend to increase with constituent molecule complexity.