ENTER_SYMBOL

This command is for the Gibbs Energy System (GES) module.

There are also a POLY and POST module commands with the same name. However, symbols entered in the GIBBS module are not the same symbols as defined in the POLY and POST modules.

In the GIBBS module, a symbol can be used to represent a numeric quantity (e.g. a variable), a function, or a table. The entered symbol (with a symbol name of maximum eight characters) can be used later when parameters are entered, and this is a flexible way to manipulate thermodynamic functions. Symbols are especially useful if the same function or table is used in several thermochemical parameters. A parameter symbol is used to assign the values of phase parameters (which are defined with the command ENTER_PARAMETER) to specific characterized symbols. If you want to define a constant quantity, enter it as a simple function with a constant value in all (temperature) ranges.

The symbols entered in the GIBBS module can also be listed and used in the PARROT module (for data optimization) where you can use the GIBBS-entered symbols (variables, functions or parameters) to define parameters that shall be optimized.

See example 44 for an example of using variables and functions.

Syntax	enter_symbol
Prompt	Variable, function, table or parameter? /Function/: <Keyword> The keyword can be a variable, function, table or parameter. `Variables` are similar to functions because these can also be expressions of state variables. It is possible to enter a variable with a new expression anytime. `Functions` are expressions of state variables or other functions. `Tables` are used for listing results. A table consists of a list of any number of state variables, functions or variables. `Parameter` symbols are used to assign the values of phase parameters (which are defined by the command ENTER_PARAMETER) to specific characterized symbols. If the phase parameter is not defined yet, the parameter symbol is assigned zero or a symbol which is valued as zero. In this way it is easy to refer to entered phase parameters in further defining other phase parameters. Name: <Name of the symbol> Each symbol has a unique name that must start with a letter and can have maximum 8 characters. The following prompts are based on the type of symbol.
`Variable` symbol	Value: <Numeric value of a variable> Only a constant numeric value, not an expression, is accepted.
`Function` symbol	Enter the lower-temperature limit, the expression, an upper-temperature limit and if there is another expression above this upper limit: Low temperature limit /298.15/: <Lowest temperature limit in K> Specify the lowest-temperature limit, below which its (first) expression is not applicable. The default lowest limit of all kinds of data is 298.15 K. If a negative number is given as the lowest-temperature limit, it is assumed that there are breakpoints in pressure for this function; in such a case, it is interpreted as the lowest-pressure limit (in Pascal), and the other limits in the current function are also taken as pressure limit values (in Pascal). The temperature/pressure limits for the functions are checked during calculations. An indicator is set if the actual temperature/pressure condition is below the lowest temperature/pressure limit or above the highest temperature/pressure limit; and in such a case, an extrapolation is conducted using the TP-FUNCTIONS valid in the nearest temperature/pressure range. Function: <Definition for a TP-function> A TP-Function consists of terms in T and P. The expression is a FORTRAN-like expression and operators +, -, , = and * can be used (** only with integer powers). Unary-functions LN or LOG (both for natural logarithm) and EXP (for exponential) can also be used. An expression can be continued on more than one line. A PT-function must be terminated by a semicolon (;) or an empty line. No more than 78 characters can be written on a line. If this is not sufficient space on one line, press <Enter> and continue on the next line. The program prompts with an ampersand `&` when a continuation is expected. A function can have several temperature ranges, and a different expression (i.e. a TP-Function) for each region; if required, a single function can have up to ten different temperature ranges (and thus have up to ten different TP-Functions). This question is repeated if the current function has more than one expression (TPFunction) in various temperature ranges [or pressure ranges; only if the lowest limit is entered as a negative number)]. & <Continuation of the definition for the current TP-Function> This is for a continuation of the definition for the current TP-Function. The ampersand & means that you can continue to write the function on the new line if one line is not enough for the current TP-Function. If you are finished the current TP-Function, press <Enter> again. High temperature limit /6000/: <High temperature limit in K> For a TP-Function, specify the high-temperature limit, above which its current expression is not applicable. The default high-temperature limit for all kinds of data is always 6000 K. This question is repeated if the function has more than one expression (TPFunction) in various temperature ranges. All the temperature limits for the presently-be-entered TP-Function MUST be in an increasing order; otherwise the whole function is given a single value of zero. If a negative number is given as the lowest-temperature limit, it is interpreted as the low-pressure limit (in Pascal), and the high-temperature limit is also taken as the high-pressure limit (in Pascal) for the current TP-Function. The high-temperature/pressure limit for the current applicable expression (TPFunction) is the low temperature/pressure limit for the next range if several ranges are needed to describe the temperature/pressure-dependence of this function. The temperature/pressure limits for the functions are checked during calculations. An indicator is set if the actual temperature/pressure condition is below the lowest temperature/pressure limit or above the highest temperature/pressure limit; and in such a case, an extrapolation is conducted using the TP-Function valid in the nearest temperature/pressure range. Any more ranges /N/: <Y or N> Answer `N` to finish the definition of the current function. If `Y` the program asks for a new function valid above the last high temperature limit and below a new high temperature limit. The maximum number of temperature ranges is 10. Important note: Make sure that the function and its first derivatives are continuous across break points.
`Table` symbol	Specify the low and high temperature limits, temperature step and tabled value at each specified temperature: Low temperature limit /298.15/: <Low temperature limit in K> The lowest temperature where the table has a value. High temperature limit /6000/: <High temperature limit in K> The highest temperature where the table has a value. Step in temperature /100/: <Temperature step> The step in temperature between each value in the table. This step must be constant for the whole table. If a table cannot be described with a single step it must be divided into several tables for different temperature ranges. The upper and lower limits are truncated to give reasonable values where the table values must be specified. Table value at XX /YY/: <Table value> The value of the table at the specified temperature (`xx`) must be given. The default value (`yy`) is the last value.
`Parameter` symbol	Input the correct name of a phase parameter (including its identifier, phase name, constituent and interacting constituent(s) in various sublattices (if exist), and degree of the parameter. Most of the settings are the same as in the ENTER_PARAMETER command or the Function symbol prompts above. Parameter: <Parameter name> Specify a correct and complete parameter name, which should contain all the necessary parts of the general form: <Identifier>(<Phase>,<Constituent array>;<Digit>) If a parameter name is not acceptable or <Enter> is pressed, this error message displays: *** Error, please re-enter each part separately The program prompts for separate input for each required part for a parameter name. Identifier (/X/): <G or L, or TC, or BM, or V0 or VA or VC or VK> Specify one of these types of legal identifiers. Phase name (/abcd/): <Phase name> Specify the phase name. Constituent (in sublattice # /abc/): <Species name> Specify the constituent name on the specified sublattice site of the given phase. Interacting constituent (in sublattice # /XYZ/): <Species name> Specify the interacting constituent name on the specified sublattice site of the given phase; if there is no interacting constituent, press <Enter>. Interacting constituent (in sublattice # /XYZ/): <Species name> If there is more than one interacting constituent on the specified sublattice site of the given phase, specify them; otherwise press <Enter>. Degree /#/: <Degree> Specify an integer number (a value from 0 through 9) as the degree for the phase parameter. Low temperature limit /298.15/: <Lowest temperature limit in K> Specify the lowest-temperature limit (in Kelvin), or the lowest-pressure limit (in Pascal but entered as a negative number), for the current TP-Function. Function: <Definition for a function> Specify the desired TP-Function for the current temperature range (or current pressure range; only if the lowest limit is entered as a negative number). & <Continuation of the definition for the current TP-function> Specify the desired TP-Function continuation for the current temperature range (or current pressure range; only if the lowest limit is entered as a negative number). High temperature limit /6000/: <High temperature limit in K> Specify the high-temperature limit (in Kelvin), or the high-pressure limit (in Pascal; only if the lowest limit is entered as a negative number) for the current TP Function. Any more ranges /N/: