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Radiation damage Chemical changes (bond breakage, ionization) caused by high-energy radiation (e.g., x-rays, gamma rays, high-speed electrons, protons, etc.).
Radical A structure with an unpaired electron (but excluding certain metal ions). In organic molecules, a radical is often associated with a highly reactive site of reduced valence (see doublet). The term radical is sometimes used to describe a substructure within a molecule; the term free radical then describes a radical in this sense, viewed as the result of cleaving the bond linking the substructure to the rest of the molecule.
Reaction A process that transforms one or more chemical species into others. Typical reactions make or break bonds; others change the state of ionization or other properties taken to distinguish chemical species.
Reagent A chemical species that undergoes change as a result of a chemical reaction.
Reagent device A large reagent structure (or a large structure that binds a smaller reagent) serving as a component of a mechanochemical system. A reagent device exists chiefly to hold, position, and manipulate the environment of a reagent moiety.
Reagent moiety The portion of a reagent device that is intimately involved in a chemical reaction.
Receptor A structure that can capture a molecule (often of a specific type in a specific orientation) owing to complementary surface shapes, charge distributions, and so forth, without forming a covalent bond. See dissociation constant.
Reconstruction A crystal consists of a regular array of atoms, and the simplest model of a crystal surface would be generated by simply discarding all atoms to one side of a surface without changing the positions of the rest. In reality, however, the positions of the remaining atoms do change. A pattern of displacements that lowers the symmetry of the surface (relative to the ideally terminated crystal) is termed a surface reconstruction; some reconstructions alter the pattern of bonds.
Reduced mass Many dynamical properties of a system consisting of two interacting masses, m1 and m2, are equivalent to those of a system in which one mass is fixed in space and the other has a mass (the reduced mass) with the value m1m2/(m1 + m2). The reduced mass description has fewer dynamical variables.
Register A temporary storage location for an array of bits within a digital logic system.
Relaxation time A measure of the rate at which a disequilibrium distribution decays toward an equilibrium distribution. The electron relaxation time in a metal, for example, describes the time required for a disequilibrium distribution of electron momenta (e.g., in a flowing current) to decay toward equilibrium in the absence of an ongoing driving force and can be interpreted as the mean time between scattering events for a given electron.
Representative point The point in a configuration space that represents the geometry of a system.
Rigid structure As used in this volume, a covalent structure that is reasonably stiff. In a typical rigid structure, all modes of deformation encounter first-order restoring forces resulting from some combination of bond stretching and angle bending; such a structure cannot undergo deformation by bond torsion alone. Meeting this condition usually requires a polycyclic diamondoid structure.


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