From: Ajasja Ljubetič (ajasja.ljubetic_at_gmail.com)
Date: Wed Oct 29 2014 - 05:07:28 CDT

Sorry, was reading too quickly.

You can use mathematical function on coordinates in the selection query
<http://www.ks.uiuc.edu/Research/vmd/current/ug/node89.html#table:ug:functions>