The Description of an Ideal Gas

We have arrived, finally, to the partition function of an ideal gas  

For monatomic gases it turns out that, with few excq)tions (Denbigh, 1981, p.379), (int) = 1.0. Thus, on a molar basis following Section 16.7  

The development of the expressions for the Helmholtz free energy 4, the Gibbs free energy G, and the entropy S is straightforward (Problems 16.16 and 16.17). 

Ideal gas entropies can also be calculated from statistical mechanics (for a monatomic gas, see Problem 16.17). Such entropy values can be used in calculating the standard entropies of reactions, because the absolute entropies of the elements drop out by virtue of the stoichiometry of 

The same, however, does not apply to the enthalpies of reaction. Here, the enthalpy of reaction at 0 K - due to the fact that the enthalpies of the compounds are relative to their values at the lowest energy level (ground state), which are not the same among different compounds - must be accounted for. 

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The simplest fitness function is the uncoupled form, in which the fitness is the sum of individual contributions from each residue, representing the special case where all mutations are additive (Aita and Hus-imi, 1996 Saven and Wolynes, 1997). This model is similar to the description of an ideal gas, where the energy contributions of the molecules are uncoupled. The fitness of a sequence F is... 

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