SET_AXIS_VARIABLE

In order to calculate a diagram, set at least one axis variable in a stepping calculation, or at least two axis variables in a mapping calculation. For property diagrams, one axis is enough; for phase diagrams two or more are necessary. Any condition that can be set to calculate an equilibrium can be used as an axis variable (with its lower and upper limits and step length) by using SET_AXIS_VARIABLE, and the POLY program does, after a STEP or MAP command, vary the value of the condition between the limits set on its related axis variable.  As an extraordinarily unique and powerful feature of the Thermo‑Calc software, up to five independent axis variables can possibly be used in a mapping calculation of a multicomponent heterogeneous system; however, the axis numbers 3, 4 and 5 must have chemical potentials of components (or temperature or pressure) as conditions.

Syntax	set_axis_variable
	Axis number /#/: <An axis number> Specify a number between 1 and 5. The axis numbers 3, 4 and 5 must have chemical potentials of components (or temperature or pressure) as conditions.
Prompts	Condition /None/: <One condition> Here the condition that should be varied along the axis must be given. The condition is specified as in SET_CONDITION, for example W(C) for mass fraction of carbon. By accepting NONE, the axis is removed.
	Min value /0/: <Min value> Specify the minimum value of the axis condition.
	Max value /1/: <Max value> Specify the maximum value of the axis condition.
	Increment /.025/: <Step length> Specify the maximum step length. By default, this is 1/40 of the total axis length.

You can give SET_AXIS_VARIABLE without having set a condition on the axis variable. Under such circumstances, the relevant condition is automatically created and the value set between the minimum and maximum axis limits; however, as a side effect, the POLY module creates two conditions, P=1e5 and N=1 (these have not been defined as a condition yet), in case you set an axis variable which is not already a condition.

You can use a logarithmic axis during calculations. This is useful for low fractions like in a gas phase where 1e-7 to 1e-2 might be an interesting range. The pressure is also suitable for logarithmic step. You specify the logarithmic axis by giving an asterisk * after the increment value.

The increment in this case is treated as a factor.

For example,

S-A-V 1 P 1E5 1E25 5*

This makes axis 1 a logarithmic axis where the difference between two calculated values makes a factor 5.

The factor must be larger than 1.0.

In some cases, such as when the DEFINE_DIAGRAM command is used or a special advanced module (e.g. BIN, TERN, POT, SCHEIL or POURBAIX) is called, some axis variables are automatically set by the program, not necessarily by this command.