Look at the difference between:
Quote:

CHARMM> open unit 21 read form name 4pti-a.pdb
VOPEN> Attempting to open::4pti-a.pdb::
OPNLGU> Unit 21 cannot be opened as 4PTI-A.PDB


and
Quote:

CHARMM> open unit 21 read form name "$CHM_STREAM/4pti-a.pdb"
VOPEN> Attempting to open::/v/estor3/home/usr/Temp/CHARMM_Tutorial/Scripts/4pti-a.pdb::
OPNLGU> Unit 21 opened for READONLY access to /v/estor3/home/usr/Temp/CHARMM_Tutorial/Scripts/4pti-a.pdb



In the first case, CHARMM translated the file name to all uppercase; I'm sure that running 'ls 4PTI-A.PDB' would return 'No such file or directory.'. Using the quotes, as in the second case, makes a difference...