Global Variables and Environment Variables

BTCL maintains some global variables that are used mainly as defaults for BTCL commands. The two most important are:

PROJECT--Used to store the current project name (also indicated here as run_name), which is used for filename completion. You may want to set this in interactive mode to test source files, for example:
```
BTCL >   set PROJECT my_project1 ; source my_project1
```
```
BTCL >   set PROJECT my_project2 ; source my_project2
```
In this example, the BTCL source codes are saved in files without the .inp filename extension.
randomSeed--This special variable is used to set the random seed for the random-number generator used by the Discover program. Setting this global variable actually causes the random number generator to be reset to this value and causes its value to be echoed, for example:
```
BTCL >   set randomSeed 35 
 randomSeed is set to 35
```
```
BTCL >   set randomSeed 
 35
```
```
BTCL >   set a [echo $randomSeed] 
 35
```

When the Discover program starts, the value of randomSeed is set randomly (according to the time of day), and its value is printed. This is so that the same random seed can be used again in later Discover runs, if desired. Setting the randomSeed variable is equivalent to using the control command of version 3.1 of DSL, a command that is no longer supported.

For commands that return the result of a numerical evaluation, the precision is controlled by the tcl_precision variable. The maximum value is 17. This value also has the special characteristic that any value converted from an IEEE floating-point value to a string and back is guaranteed to have the same binary representation before and after the conversion.

The Discover program makes use of several environment variables. These are easily accessed by BTCL by using the $env() array. For example, the $FORCEFIELD environment variable can be accessed by:

BTCL >   echo $env(FORCEFIELD) 
 cvff

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