In chapter 2 it was shown how the storage of thermal energy can be understood on a quantum theoretical basis. 
The thermodynamic relationship between the molar entropy σ_{m} and the molar thermal capacity C_{p} can best be seen in the relationship given below

Since the logarithm function is monotonically increasing, the term ln(τ) also increases with increasing temperature. If the entropy σ is semilogarithmic against the temperature, the thermal capacity can be determined as the slope (first derivative) in this diagram. Thermodynamics distinguishes four types of quantized energy storage. Because the chemical substances have four kinds of eigenvalues, there are also four types of energy stores, which belong to four types of energy states: translational, rotational, vibrational, and electron states. They behave differently when thermal or volume work on the substances is carried out. The fluid model is capable of adequately visualizing the different behavior, because it is able to convert quantumchemically calculated values of the individual memories graphically. The ThermulationII program calculates these values from the known spectroscopic data and can output these as numerical values, as Boltzmann distributions and as fluid model diagrams. 
Thermodynamically, the temperature is defined by the relation given below. 

A vivid idea of this definition is obtained by the following storage analogy: 
It corresponds: 
To prove this we show that the hydrostatic pressure p_{hyd} is defined analogously to the temperature: 
The outlet line  possibly with a tap  represents the (thermal) contact to the environment or to other substances. A closed tap would mean perfect thermal insulation. 