title: Why can't I start pymol?
body: <p>I'm trying to run the biotech software pymol. It can't find the <code>cmd.py</code> module. However, it's in a directory, which is in the $PYTHONPATH.</p>
  
  <pre><code>[mzhKU_work] @ modules $ python launch_pymol.py 
  Traceback (most recent call last):
    File "launch_pymol.py", line 32, in &lt;module&gt;
      import pymol
    File "/Users/mzhKU_work/software/pymol/modules/pymol/__init__.py", line 521, in &lt;module&gt;
      import pymol._cmd
  ImportError: No module named _cmd
  
  [mzhKU_work] @ modules $ ls ./pymol/
  __init__.py          exporting.py         opengl
  __init__.pyc         externing.py         parser.py
  api.py               feedingback.py       parsing.py
  callback.py          fitting.py           plugins
  cgo.py               gui.py               povray.py
  checking.py          gui.pyc              preset.py
  cmd.py               headering.py         querying.py
  cmd.pyc              helping.py           rpc.py
  colorramping.py      importing.py         selecting.py
  commanding.py        internal.py          selector.py
  completing.py        invocation.py        setting.py
  computing.py         invocation.pyc       shortcut.py
  constants.py         keyboard.py          util.py
  constants_palette.py keywords.py          vfont.py
  contrib              locking.py           viewing.py
  controlling.py       m4x.py               wizard
  creating.py          menu.py              wizarding.py
  editing.py           monitoring.py        xray.py
  editor.py            morphing.py          xwin.py
  embed                movie.py             xwin.pyc
  experimenting.py     moving.py
  
  [mzhKU_work] @ modules $ echo $PYTHONPATH
  [...]/Users/mzhKU_work/software/pymol/modules:/Users/mzhKU_work/software/pymol/modules/pymol:/Users/[...]
  [mzhKU_work] @ modules $ 
  </code></pre>
  
  <p>What could be the problem?</p>