About Alternate Mode

To turn Alternate Mode ON or OFF, use the PARROT command SET_ALTERNATE_MODE.

When you have one or more multi-phase equilibria (that is, with the fixed phase-status) in your POP file and the optimizing values lack appropriate starting values (such as the default value of 0 for example), then Thermo‑Calc may not be able to compute the equilibria for the experiments. In these cases, a common tangent cannot be found, that is, a state where the chemical potentials for all components are the same in all phases cannot be found. A common tangent may not exist or it be found at the wrong composition or temperature given the initial set of model parameters.

If you are not able to estimate reasonable starting values for the optimizing variables, then this problem can be overcome by initially running your optimization in alternate mode. The alternate mode in PARROT module should help you find reasonable starting values for the optimizing variables. Note that you are only supposed to use alternate mode in the beginning of the optimization. Once you have achieved a reasonable fit between calculated results and experimental data, turn off alternate mode and calculate all equilibria again in the normal mode.

In alternate mode, PARROT modifies multi-phase equilibria so that the chemical potentials for each stable phases is calculated separately. For example, consider the following experimental equilibrium:

CREATE_NEW_EQUILIBRIUM 1 1

CHANGE_STATUS PHASE LIQ FCC=FIX 1

SET_CONDITION T=1000 P=1E5

EXPERIMENT X(LIQ,B)=.2:.01 X(FCC,B)=.1

When PARROT optimizes in alternate mode, the computation of this equilibrium is done by way of the following two calculations:

CHANGE_STATUS PHASE LIQ=FIX 1

SET_CONDITION T=1000 P=1E5 X(LIQ,B)=.2

CHANGE_STATUS PHASE FCC=FIX 1

SET_CONDITION T=1000 P=1E5 X(FCC,B)=.1

The difference between the chemical potentials in every pair of phases is calculated and PARROT tries to adjust the parameters of the phases so that these differences are minimized. In other words, PARROT tries to make the chemical potentials equal in all phases. For PARROT to be able to do this, some additional information about the experimental multi-phase equilibria is required, at least for all invariant equilibria.