Equilibrium cooling, heating

Figure 2-2 Calculated cooling-heating behavior for a second-order reversible reaction. The equilibrium and kinetic parameters are from Reaction 2-55. From Zhang (unpublished).

However, more recent work has shown this idea to be untenable. There are many spontaneous endothermic reactions—for example, the solution in water of salts such as lithium chloride, where there is a cooling of the system. Furthermore there are many examples in which a reaction can be made to proceed in either direction to a definite state of equilibrium. Therefore heat evolved is not the sole driving force, although it does make an important contribution to the driving force. 

Consider how the change of a system from a thennodynamic state a to a thennodynamic state (3 could decrease the temperature. (The change in state a —> f3 could be a chemical reaction, a phase transition, or just a change of volume, pressure, magnetic field, etc). Initially assume that a and (3 are always in complete internal equilibrium, i.e. neither has been cooled so rapidly that any disorder is frozen in. Then the Nemst heat... 

Force field calculations often truncate the non bonded potential energy of a molecular system at some finite distance. Truncation (nonbonded cutoff) saves computing resources. Also, periodic boxes and boundary conditions require it. However, this approximation is too crude for some calculations. For example, a molecular dynamic simulation with an abruptly truncated potential produces anomalous and nonphysical behavior. One symptom is that the solute (for example, a protein) cools and the solvent (water) heats rapidly. The temperatures of system components then slowly converge until the system appears to be in equilibrium, but it is not. 

Amolecular dynamics simulation can have three distinct time and temperature periods heating, simulation (run), and cooling. If you want to measure equilibrium properties of a molecular system, you can divide the simulation period into two parts equilibration and data collection. 

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