ASSESSMENT module provides tools to help in the task of deriving
values for thermodynamic model parameters, which, when used as a basis
for phase equilibrium calculations, give the best possible
representation of a collection of experimental measurements obtained
for a particular chemical system. This overall task of gathering data
and generating values for parameters is referred to as data assessment
or sometimes critical assessment since it involves comparing data of
many different types, from many different sources, obtained using many
different experimental techniques. The specific task of generating
model parameters appropriate to a set of experimental data is called
data for a particular system are supplied to the ASSESSMENT module via
an experimental data file. Any property or combination of properties
that can be calculated using MTDATA can feature in this file, which
takes the form of a macro giving instructions for the calculation of
each property and associating each calculation with an experimental
value and uncertainty (using the LIST command). Properties typically
considered in experimental files include activities, heats of mixing,
equilibrium phase compositions and phase boundary temperatures and
compositions. Best results are obtained where a good mix of
thermodynamic measurements and phase equilibrium data are available.
thermodynamic parameters from the data input (.mpi) file are referred
to by number (or index) during the process of setting up an
optimisation run. The SHOW command is used to reveal the index
appropriate to each parameter. Once a parameter's index is known the
parameter can be flagged for possible change during an optimisation run
using the MODIFY command. Sets of parameters can also be linked
together such that any change made to one will be mirrored in its
partner(s). This is useful in ensuring that data remain consistent
across temperature range boundaries.
A optimisation run based upon a chosen algorithm can be initiated using
the OPTIMISE command. During such a run the values of parameters
flagged for optimisation are changed automatically to minimise
differences between calculated properties defined in the experimental
data file and their experimental values. The actual function minimised
is the weighted sum of squared errors where an error is defined as the
difference between calculated and experimental values of a property
divided by the stated experimental uncertainty.
n is the number of properties considered in the optimisation run, Ci
is the calculated value of property i, Ei is the
experimental value of property i, Ui is the
uncertainty associated with Ei and Wi
is the weight assigned to property i.
runs can be monitored graphically or by means of text output. In the
former case plots illustrate changes in the optimised parameters and
corresponding changes in the weighted sum of squared errors as they
happen. Typically an assessment will involve many optimisation runs
with different numbers of parameters being changed and more
experimental data being added at each stage.
a prelude to an optimisation run it is often useful to obtain a crude
impression of the effects of varying individual thermodynamic
parameters. This helps in selecting suitable parameters for change. To
facilitate this, one or two parameters can be flagged for stepping
using the MODIFY command. On issue of the STEP_PARAMETER command the
values of these parameters are varied automatically based upon a
specified SCALE (magnitude of changes) and number of steps, a
comparison between calculated properties and experimental data being
made at each stage.
calculated using parameter values resulting from an optimisation run
can be compared with experimental data graphically using the PLOT
command or in the form of a results table using the LIST command. In
order to make more complex calculations using the new parameters it is
possible to SWAP to other MTDATA modules while still retaining the
optimised data in memory. If the new parameters are found to be
unsatisfactory, perhaps due to a lack of experimental data to constrain
their values, it is possible to UNDO the effects of an optimisation run
and recover the parameter values appropriate at its start.
values of parameters can be saved in an in-memory store or saved in a
data loading file for the purposes of updating an MTDATA database.
Alternatively a set up file (MTDATA macro) can be written, allowing the
exact state of the existing optimisation to be recreated at a later
Brief summary of commands
is used to retrieve thermodynamic data for a system to be studied,
either from a list of databases or from a previously created data input
(.mpi) file. Additionally it is used to provide the name of the file
(macro) containing calculation instructions and experimental data for
any planned optimisation runs.
LIST provides the basic tools for setting up an
optimisation run in that it allows target experimental values and
uncertainties, labels, weights and suitable abscissa settings (for
later plotting) to be associated with properties to be calculated. It
can also be used, after optimisation, to examine thermodynamic
parameter values and differences between calculated properties and
experimental data. Alternatively it displays the elements, components,
substances, unaries and phases in the currently defined system, their
status (classifications) and any initial or equilibrium settings
(including ranges) established previously.
CLASSIFY is used to remove particular components, phases or
substances from consideration in calculations or to reintroduce those
previously removed, to establish reference states for the thermodynamic
properties of components, to flag "PARA" components, the relative
amounts of which are to be fixed in all phases in which they appear,
and to indicate potential miscibility gaps in phases. It is also allows
computational parameters appropriate to open system calculations to be
specified and the amounts of components to be set free to change.
MODIFY_PARAMETER is used to assign new absolute or relative values
to thermodynamic parameters, to flag those parameters to be changed
during stepping or optimisation, to link parameters together so they
are changed in an identical manner and to provide scales for changes to
RANGE is used to specify temperature, pressure or
composition ranges for consideration in phase boundary searches along
with temperature, pressure or composition tolerances indicating the
accuracy to which individual boundaries are to be found. Note that
currently only temperature can be ranged.
SET is most commonly used to fix the system
temperature, pressure or volume and initial amounts or masses of
components, individually or in total, prior to a calculation. It can
also be used to fix equilibrium partial pressures, molalities and
amounts or masses of individual species, to associate group names with
system compositions and to assign values to named user variables.
STEP_PARAMETER is used to initiate calculations in which the
values of one or two thermodynamic parameters are stepped
automatically, the level of agreement between calculated properties and
experimental data from the current experimental data file being
indicated at each stage in terms of the weighted sum of squared errors
divided by the sum of weights. This is sometimes a useful prelude to a
full optimisation run. The required number of steps must be specified.
SHOW lists the names, current values and indices of
thermodynamic parameters from the current data input (.mpi) file to the
screen. Parameters can be selected for listing by type (unary or
interaction) or by phase.
PLOT is used to obtain diagrams comparing calculated
properties with experiment. Two types of diagram can be plotted, bar
charts of errors (differences between calculated and experimental
results divided by experimental uncertainties) or lines joining
calculated values associated with the same abscissa variable overlaid
with symbols indicating experimental data. Weights are not shown in
SAVE is used to save the values of fixed parameters,
parameters flagged for change during stepping or optimisation and
parameters linked to others flagged for change, either to an in-memory
store or to a data loading file ready for creating or updating an
MTDATA database. Alternatively a set up macro can be written allowing
the exact state of the current optimisation to be recreated at a later
COMPUTE is used to initiate phase equilibrium calculations
based upon the current settings and classifications. The required type
of calculation (stage or combination of stages) and method of
initiation (initial point option) can be chosen.
OPTIMISE is used to initiate an optimisation run in which
thermodynamic parameters flagged as optimised or linked are adjusted
automatically to minimise differences between calculated properties and
experimental data from an experimental data file. An appropriate
optimisation algorithm, tolerance, maximum number of comparisons to be
made between calculation and experiment and method of monitoring
progress (text or graphical output) can be selected.
RESTORE is used to retrieve the values of fixed
parameters, parameters flagged for change during stepping or
optimisation and parameters linked to others flagged for change, from
in-memory store. Existing values are overwritten.
UNDO_OPTIMISATION negates the effects of the last optimisation run
performed. New values for thermodynamic parameters generated during the
last optimisation run are overwritten by those current at the start of
LASER is used on some MTDATA platforms to request hard
copies of graphs plotted but not previously sent to a printer. It is
not used in MTDATA for Windows 9x and NT.
takes the user out of FIT_AND_PLOT back to the
is used to leave the ASSESSMENT module and enter
another module, retaining the current optimisation set up. This allows
test calculations to be performed in modules other than ASSESSMENT
using the current set of optimised parameters.
runs a macro taken from a file. The name of the file should be
entered in quotes. Commands can also be passed through to the
operating system by prefixing them with a $ (dollar sign). The
dollar sign and operating system command should be entered within
A gentle introduction to using
the options available in ASSESSMENT contains
examples, each directed towards achieving a particular outcome
and shows typical combination of commands which would be necessary
to achieve that end.
A simple summary of the commands
available can also be used as an index to obtain more detailed
help about each command.