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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).
$EXISTsymbol-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 (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).