Symbols

A symbol is a word with a “$" as the first character. It is replaced by another word that has been assigned by the user or by X-PLOR during program execution.

$<$symbol$>$:==

includes symbols that can be assigned to an arbitrary value and manipulated by the evaluate statement (see Section 2.14) and several control statements (see Section 2.8). The data type can be real, string, or complex and is assigned automatically by the program. The name of the symbol is a string that qualifies as a word with fewer than 20 characters. X-PLOR internally declares particular symbols when certain statements are executed. Here are a few examples:

$?

is not an assignable symbol; it produces a list of the currently assigned symbols.

$ANGL,

$BOND, $DG, $DIHE, $ELEC, $ENER, $HARM, $IMPR, $NCS, $NOE, $PELE, $PLAN, $PVDW,$RELA,
$VDW, $XREF, represent partial energy terms (see Sections 4.5 and 4.6). The data type is real. These symbols are declared upon evaluation of the X-PLOR energy function.

$CPU

contains the CPU time (real).

$DATE

contains the current date (string).

$EXIST $<$symbol-name$>$

is not an assignable symbol; it is a special symbol that returns the string “TRUE" if $<$symbol$>$=$<$symbol-name$>$ has been declared previously; otherwise it returns “FALSE".

$GRAD

specifies the rms value of the energy gradient. The data type is real. This symbol is declared upon execution of the X-PLOR energy function.

$KBOLTZ

is declared during X-PLOR start-up. It contains the Boltzmann constant in units of the program (real).

$NAME

is declared during X-PLOR start-up. It contains the username (string).

$PI

is declared during X-PLOR start-up. It contains the value of $\pi$ (real).

$RESULT

contains the results after execution of various statements, such as “VECTor SHOW" and “COORdinates RMS END". Data type is either real or string, depending on the application.

$STATUS

is declared during X-PLOR start-up. It provides status information about the read dynamics statement (string).

$SYSTEM

is declared during X-PLOR start-up. It contains the system identification (string).

$TIME

contains the wall-clock time (string).