MTDATA – Phase Diagram Software from the National Physical Laboratory

 

 

 

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COMPUTE

Function:

This command is used to initiate a phase equilibrium calculation  based upon the current settings (see SET) and classifications (see CLASSIFY). The required type of calculation (STAGES_REQUIRED and INITIAL_POINT_OPTION) can be specified. Note that this command does not initiate an optimisation run (see OPTIMISE).

Parameters:

initial_point_option

stages_required

Defaults:

initial_point_option = automatic

stages_required = 1_only

Examples:

compute !

compute stages = both_small initial = last !

Parameter values:


INITIAL_POINT_OPTION

takes the values AUTOMATIC / LOWER_BOUNDS/ PREVIOUS_SOLUTION / LAST_ACTIVE_SET

The INITIAL_POINT_OPTION parameter is used to specify how initial values for properties such as equilibrium substance amounts are to be estimated. These initial values are altered by the program during the course of a phase equilibrium calculation to achieve the lowest possible overall Gibbs energy.

Usually it is best to accept the default INITIAL_POINT_OPTION, namely AUTOMATIC, and let the program choose its own initial values. Sometimes, however, it is beneficial to use results from one calculation to provide initial values for another. This is done by entering INITIAL_POINT LAST_ACTIVE_SET or INITIAL_POINT PREVIOUS_SOLUTION. A typical use of these options is to speed up calculations in a closely related series. 

AUTOMATIC specifies that the program should select its own initial values.

LOWER_BOUNDS specifies that initial amounts should be set at their lower bounds, effectively zero.

PREVIOUS_SOLUTION specifies that the equilibrium substance amounts produced by one calculation should be used as initial values in the next.

LAST_ACTIVE_SET similar to PREVIOUS_SOLUTION but also makes available intermediate variables from a preceding STAGE_1 calculation.

STAGES_REQUIRED

takes the values 1_ONLY / STAGE_1_PLUS / 2_ONLY/ ROBUST_STAGE_2 / BOTH_LARGE / BOTH_SMALL

The STAGES_REQUIRED parameter is used to select the algorithm to be used in performing phase equilibrium calculations.

1_ONLY  selects the STAGE_1 Gibbs energy minimisation algorithm, which gives equilibrium compositions to six decimal places in amounts. STAGE_1 is the only algorithm available for certain thermodynamic models, including the compound energy model with multiple sublattices. It should not be used when the amounts at equilibrium of more than two substances are constrained and is not suitable for calculating amounts of less than 10-6 moles.

STAGE_1_PLUS  selects the STAGE_1_PLUS algorithm which effectively carries out a highly robust STAGE_1 calculation followed by a STAGE_2 calculation for certain phase types, such as gas. This produces a refinement of the composition of such phases.

2_ONLY  selects the STAGE_2 algorithm, which is not as robust as STAGE_1 and can fail unless used in conjunction with STAGE_1 (BOTH_LARGE, BOTH_SMALL, ROBUST_STAGE_2) or, with an appropriate INITIAL_POINT_OPTION, following a preceding calculation with very similar settings. STAGE_2 gives equilibrium compositions to six decimal places in logarithms of amounts. Its applicability is limited to certain models.

ROBUST_STAGE_2  selects the ROBUST_STAGE_2 algorithm which is an intelligent combination of STAGE_1 and STAGE_2 involving repeated STAGE_2 calculations interspersed with STAGE_1 calculations if difficulties arise. This is useful for performing sets of related calculations in systems with large numbers of species, but should not be used if the number of solution phases at equilibrium is liable to change.

BOTH_LARGE selects an algorithm involving the automatic use of STAGE_1 followed by STAGE_2, any equilibrium constraints being applied in both cases. Its use is appropriate where equilibrium constraints on amounts are of the order of 10-5 or above and two in number, at most. Otherwise BOTH_SMALL should be used. STAGE_2 limitations on models apply.

BOTH_SMALL  selects an algorithm involving the automatic use of STAGE_1 followed by STAGE_2 with any equilibrium constraints being applied just for STAGE_2. Its use is appropriate where equilibrium constraints on amounts are of the order of 10-5 or below or number more than two.  STAGE_2 limitations on models apply.

 

Updated 4 August 2010