Thermodynamic calculations for complex systems

A complex system is a system subject simultaneously to several equilibria, which may be physical equilibria of state change and chemical 

To calculate a system is to give, in specific conditions - i.e. of temperature, pressure, concentrations of certain species, and initial conditions - the nature and composition of the phases in equilibrium. 

The calculation requires a clear definition of the system, and the result obtained is valid only in the strict context of that definition. 

The system is defined by the exhaustive list of the compounds and phases which the operator takes into consideration, and the choice of constraints s/he imposes, such as the temperature, the pressure or the domains of temperature, pressure, the concentrations or the initial quantities. 

This definition is highly important, as no computation method can show the existence of a compound in a specific phase which the operator has not chosen in the definition list. 

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Often thermodynamic calculations for complex systems are made assuming that all chemical changes can take place that are allowed within the framework of the atomic material balances. This approximation may be appropriate at high temperatures but is often not true for catalytic systems. 

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