Hi,
I tried your suggestion, get the same error (see error below). Here are the different tests I've tried, only 'Test 3' works.

* test
*

! Test 0
!stream "4pti-a.pdb"
!stop

! Test 1
!open unit 21 read form name 4pti-a.pdb
!close unit 21

!Test 2
!open unit 21 read form name /v/estor3/home/usr/Temp/CHARMM_Tutorial/Scripts/4pti-a.pdb
!close unit 21

! Test 3
open unit 21 read form name "/v/estor3/home/usr/Temp/CHARMM_Tutorial/Scripts/4pti-a.pdb"
close unit 21

Here is my computer info:
Linux de6 2.4.21-4.ELsmp #1 SMP Fri Oct 3 17:52:56 EDT 2003 i686 i686 i386 GNU/Linux

I am running charmm version c30b2.

Ed

Here was the error message for 'Test 0':
CHARMM> ! Test 0
CHARMM> stream "4pti-a.pdb"
VOPEN> Attempting to open::4pti-a.pdb::
OPNLGU> Unit 99 cannot be opened as 4pti-a.pdb

***** LEVEL 0 WARNING FROM *****
***** "OPEN" not possible.
******************************************
BOMLEV ( 0) IS REACHED - TERMINATING. WRNLEV IS 5