CHARMM Development Project Forums User Discussion & Questions Setup, I/O, and Basic questions Construct "##IF KEY (key)"

Hi,
I was reading charmm source code, and got a little confused by the preprocessing construct "IF KEY (key)". Here is the documentation in prefx.f:
C-----------------------------------------------------------------------
C - Allows control line identifiers. For example:
C ##IF PERT (pertprint)
C ...
C ##ELSE (pertprint)
C ...
C ##ENDIF (pertprint)
C This construct checks the identifiers and prints a fatal
C error message if the identifiers do not match.
C-----------------------------------------------------------------------
Does it mean that it will check if PERT is in the keyword list, and will print "pertprint" if "PERT" is not in the list? Thanks!

No.
It means that if PERT is in pref.dat the code between ##IF PERT and ##ELSE will be executed, else the code between ##ELSE and ##ENDIF is executed.

Yeah, I know that is true for "##IF KEY" construct. Then what's the meaning of stuff in the parenthesis, i.e. "pertprint"?

Check for programmer errors.

Found the answer in developer.doc:

match-token :: unique text string in parentheses; must be the
same for each use in an ##IF ... ##ENDIF block

ie, to check for programmer errors.