A representative collection of heat content plus zero-point energies deduced in the harmonic oscillator approximation suggests a number of simple additivity rules which allow ZPE + (HT -Ho) to be related to structural features. Accurate evaluations are obtained for alkanes, alkenes, alkynes, cycloalkanes, chloroalkanes, and amines, as well as for large and small cycloalkanes in which CH2 is replaced by 0 . IntroductionIn our endeavour to understand the physics governing the energy of a molecule, traditional quantum mechanical energy minimization ends up describing a molecule at equilibrium, at its potential minimum. Thermochemistry, on the other hand, offers energy data (e.g., standard enthalpies of formation) for molecules possessing vibrational, rotational, and translational energies. This is where the problem lies. Vibrational energies are usually not as readily available as desired, thus rendering bona-fide comparisons between theory and experiment difficult, in spite of a wealth of thermochemical information. Indeed, the spectroscopic determination of the required complete sets of fundamental vibrational frequencies is usually far from being trivial, hence our interest in alternate rules which would allow the construction of reliable vibrational energies in a simple manner.The sum of zero-point (ZPE) plus heat content (HT -Ho)