In reality, the molecules are not the inert rigid masses. The forces of attraction and repulsion which we ascribe to them are in reality the consequence of variations in the quantum states of a deformable electron cloud which fills practically all the space occupied by a molecule so that when we represent it as a rigid mass we are constructing a model which allows us to apply classical mechanics to relate its energy changes to changes in its microstate properties. For example, an effective model for a complex molecule is to regard it as a group of rigid spheres of various size and mass held together by flexible springs. The only justification for this model is that calculations of its energy, when properly averaged, give good agreement with values of energy per molecule obtained from experimental measurements using bulk quantities of the substance.

Constructing models is important in all aspects of thermodynamics, not only for individual molecules, but also in describing the behavior of bulk matter. Values, which can be calculated from the microstate properties of an individual particle or of a cluster containing only a few particles, represent another group of non-thermodynamic properties. We will refer to these derived values as "molecular" properties. Examples are the translational, vibrational, or rotational energies of an individual molecule, and also the calculated potential energy at various separation distances in a pair of molecules or between other small groups of near neighbors. In some cases we wish to calculate special functions of the potential energy within a group composed of a few neighbors. An important feature of all of these combinations of fundamental microstate properties is that they can produce the same value of a calculated molecular property. For example, assigning values to the microstate properties of a molecule determines its energy but specifying the energy of a molecule does not specify any one particular set of values for its microstate properties.

Whereas the non thermodynamic properties pertain to a single or to only a few ultimate particles, the characteristics of matter which are called thermodynamic properties or macrostates are those which result from the collective behavior of a very large number of its ultimate particles. Instead of only one or a few particles, this number is typically on the order of Avogadro's number. In a manner analogous to the way in which molecular properties can be calculated from the fundamental microstate properties of an individual or small group of particles, the various thermodynamic properties likewise depend upon the vastly greater number of all the microstate properties of the very large group. Furthermore, an even larger number of different sets of microstate properties can produce the same overall thermodynamic property value. In contrast to non-thermodynamic properties, thermodynamic properties can always be measured experimentally or calculated from such measurements.