Elementary considerations permit the pressure derivatives of the compressibility and of the dielectric constant to be related to more easily measurable quantities. Quantum mechanics, for example, the theory of the hydrogen-molecule ion, shows that the bonding and antibonding states of a covalent bond are simply related. Thus, optical energy gaps and compressibilities can be combined to define the energetics of the bonds. Values of pressure derivatives thereby predicted compared favorably with experiment. © 1975 The American Physical Society.