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## In this section:

• How to define your system
• How to calculate the equilibrium and the paraequilibrium constituents distribution

## Defining the system

Let us imagine you want to know what are the equilibrium phases in a Fe-Cr-Ni-C system which composition would be 18/12/0.05, Fe being the balance to 100.

 You type (+enter) The prompt is Comments WHICH MODULE ? multiphase MULTIPHASE OPTION ? You entered the multiphase  module (more about modules) define DEFINE WHAT ? You can have a look a all the possibilities by typing simply ? source ENTER LIST OF DATABASE NAMES : The same ? will give you the list of sources you can use (more about databases). plus ! MULTIPHASE ? The ! marks the end of the command ( more about general command structure).

Now you can define the system in a similar way:

define system 'Fe,Cr,Ni,C' !

MTDATA goes through the different databases you selected and keeps all the phases which can be formed by the elements you gave. Because the variable [MISSING_DATA is set to CONTINUE, MT_DATA will not stop if parameters are missing, but you will find the phase initially classified as ABSENT.

Because there are not too many phases in this example, you can use them all:

classify normal p(*) !

## Settings the system conditions

We still can't do a lot. We need to set the amount of each component and the temperature. There is more than one way to do this, the easier is described:
First, you want to set the total mass of your system to 100kg, in a way you can directly use the weight percents which define your system:

set w 100 !

Now, defining the amount of each component :

set w( )

Note that you need to leave a space between the two parentheses. This will cause mtdata to ask you the amount of each component. Since you don't want to bother calculating what is the amount of Fe in the system, just enter undef when it asks you about the mass of Fe. It will calculate the amount of Fe to balance.
Once you've finished (ie you've entered the amount of C), you see the prompt DEFINE WHAT ? This is because you didn't type the ! which will cause mtdata to execute the command you just entered.

Is just left the temperature:

set temperature 973 !