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.
