It appears the file is readable/writable

m1~/Temp/CHARMM_Tutorial/Scripts/>ls -ls *pdb
48 -rw------- 1 ed cbs 46656 Oct 23 2003 4pti-a.pdb
8 -rw------- 1 ed cbs 4861 Oct 23 2003 4pti-wat.pdb
20 -rw------- 1 ed cbs 19582 Oct 23 2003 atcg.pdb

so I don't think that is the problem.

I modified the permissions to let all users have read/write access
m1~/Temp/CHARMM_Tutorial/Scripts/>chmod a+rw 4pti-a.pdb
m1~/Temp/CHARMM_Tutorial/Scripts/>ls -ls *pdb
48 -rw-rw-rw- 1 ed cbs 46656 Oct 23 2003 4pti-a.pdb

I then rerun the original script, only to get the same error:
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 Bogdan you are right, the capitalized form of the file does not exist:
m1~/Temp/CHARMM_Tutorial/Scripts/>ls 4PTI-A.PDB
ls: 4PTI-A.PDB: No such file or directory

Any other ideas what might be causing this problem?
Thanks for your help,
Ed