Dear Chimera users, The command I used was chimera --nogui test.py I tried two codes (following previous posts and tutorials in chimera) in 'test.py' as follows (i) import os from chimera import runCommand as rc from chimera import replyobj rc("open " + 'test.pdb') for i in [2011,2420]: rc("""distance #0.2:2010.B@CB #0.1:%d.B@CB""" %i) *This code gave the result as expected*: Executing test.py... Opening test.pdb... 60 models opened Distance between #0.2:2010.B@CB and #0.1:2011.B@CB: 72.346 Distance between #0.2:2010.B@CB and #0.1:2420.B@CB: 92.979 Executed test.py (ii) import os from chimera import runCommand as rc from chimera import replyobj rc("open " + 'test.pdb') for i in range(2011,2420): rc("""distance #0.2:2010.B@CB #0.1:%d.B@CB""" %i) *This code gave the result as* ' You selected %d.' % numSelected) MidasError: Exactly two atoms/axes/planes must be selected. You selected 1. Error while processing test.py: MidasError: Exactly two atoms/axes/planes must be selected. You selected 1. File "/opt/UCSF/Chimera64-1.5.3/share/Midas/__init__.py", line 1266, in distance ' You selected %d.' % numSelected) What could be the error this code? Only difference between two codes is: "for * i in [2011,2420]:" & "for i in range(2011,2420):"* That is, I am interested to use range operator in 2nd case instead of two values as in the 1st case. Thank you for help, Mahendra Thapa University of Cincinnati,OH