to the spatial relationships of atoms in
structure, and in particular, to the space-filling properties of a molecule. If molecules
were rigid and had hard surfaces, steric properties would merely be an opaque
way of saying "shape"; a flexible side-chain, however, has definite
steric properties but no fixed shape. Nanomechanical systems
make extensive use of the steric properties of relatively rigid molecules, for which the term "shape" has
essentially its conventional meaning so long as one remembers that the
surface interactions are soft on small length-scales.