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MODIFY_PARAMETER

Function:

The MODIFY command is used to select thermodynamic parameters from the current data (.mpi) file the values of which are to be FIXED, OPTIMISED, LINKed or  STEPPED, to assign new absolute or relative (OFFSET) values to parameters, to guide the optimisation process by providing SCALEs for any changes made to parameters (also used as step sizes during parameter stepping) and to LINK parameters together so that any changes made to one will be reflected in its partner(s).

Parameters:

index

link

offset

scale

type

value

Defaults:

scale = determined by the program

type = fixed

value =parameter value from the current data (.mpi) file (with no offset)

Examples:

modify index 123 type optimise offset -10000 !

mod ind 12 value -154749.35 !

modify index 15 link 9 !

modify index 86 type opt offset -5000 scale 1000 !

modify index 85 type fixed offset -10000 !

modify index 19 type unchanged !

Parameter values:

INDEX

takes the value <number>

MODIFY INDEX is used to select a thermodynamic parameter from the DEFINEd data input (.mpi) file. The value of this parameter, or its intended status during subsequent optimisation and stepping runs, can be changed using other keywords entered as part of the same MODIFY command (TYPE, VALUE, LINK, OFFSET, SCALE). The INDEX keyword should be followed by the number of the selected parameter obtained using the SHOW command which lists parameters available for change and their corresponding numbers.

LINK

takes the value <number>

MODIFY LINK is used to set up groups of LINKed thermodynamic parameters such that changes made to one parameter in a group are reflected in the others. This is useful, for example, in ensuring that equivalent parameters appropriate to different temperature ranges within a single data set are treated identically during optimisation runs. The number of the parameter for which a LINK is to be established should be given after the INDEX keyword and the number of the parameter it is to be LINKed to, the target parameter, should be given after the LINK keyword. For example:
       
modify index 17 link 11 !

LINKs parameter 17 to parameter 11. Parameter numbers can be viewed using the SHOW command. The parameter with the lowest number within a LINKed group should act as the target for the others. More than one parameter can be LINKed to the same target, but LINKed parameters cannot, themselves, act as targets. Target parameters must be OPTIMISED, STEPPED or FIXED (see MODIFY TYPE).

OFFSET

takes the value <user supplied value>

MODIFY OFFSET is used to change the value of a thermodynamic parameter, selected using MODIFY INDEX, by applying a specified OFFSET to the number found in the data input (.mpi) file. According to the TYPE of the parameter in question (see MODIFY TYPE) this OFFSET might be FIXED or subject to automatic change during optimisation (OPTIMISED) or stepping (STEPPED). The OFFSET keyword should be followed by the numeric value of the required OFFSET. For example:

modify index 24 offset -10000 !

SCALE

takes the value <user supplied value>

MODIFY SCALE is used to associate a SCALE with a thermodynamic parameter selected from the current data input (.mpi) file using MODIFY INDEX. A parameter’s SCALE is used as a basis for determining the magnitude of changes to be made to its VALUE during optimisation and stepping. The SCALE required should be entered as a positive number after the keyword SCALE. For example:

modify index 36 value  -10000 scale 1000 !
    
It is often best to let the program determine its own SCALE for changes to thermodynamic parameters made during optimisation. This is done by default or if a parameter’s SCALE is set to zero.

In the output produced during optimisation runs (see OPTIMISE OUTPUT_TYPE) changes in parameter values are expressed as fractions or multiples of corresponding parameter SCALEs. This allows changes associated with different types of parameter to be compared on common basis.

TYPE

takes the values FIXED / STEPPED / OPTIMISED / UNCHANGED

MODIFY TYPE is used to indicate the intended status during stepping or optimisation runs of thermodynamic parameters selected using MODIFY INDEX. Parameters can be assigned fixed values (FIXED), flagged for automatic change during stepping runs (STEPPED) or flagged for automatic change during optimisation runs (OPTIMISED). Previous TYPE settings can be cancelled using the UNCHANGED keyword.

FIXED indicates that the value of the selected thermodynamic parameter should remain FIXED during subsequent stepping and optimisation runs. It is often used in combination with MODIFY VALUE to assign a new fixed value to a chosen parameter. For example:

modify index 31 type fixed value -18456 !

STEPPED flags the selected thermodynamic parameter such that, when the STEP_PARAMETER command is issued, its value is STEPPED automatically between limits determined by its SCALE, to determine the effect on calculated properties (see LIST PROPERTY) from an EXPERIMENTAL_FILE (see DEFINE). No more than two parameters can be STEPPED at one time.

modify index 7 type stepped scale 5000 !

OPTIMISED flags the selected thermodynamic parameter for automatic change  during future optimisation runs. For example:

modify index 21 type optimised !

UNCHANGED negates the effects of previous OPTIMISED, STEPPED or FIXED entries for the selected thermodynamic parameter. The value of the parameter is returned to that in the data input (.mpi) file.

VALUE

take the value <user supplied value>

MODIFY VALUE is used to specify a new value for a thermodynamic parameter selected from the current data input (.mpi) file using MODIFY INDEX. According to the TYPE of the parameter in question (see MODIFY TYPE) this VALUE might be FIXED or subject to automatic change during optimisation (OPTIMISED) or stepping (STEPPED). The VALUE keyword should be followed by the new number required. For example:

modify index 24 value 1427346.34 !

 
General comments:

None.

 

Updated 30 July 2010