Advice - drawing structures under X?

[Raul Valdes-Perez <valdes %! at !% CARMEN.KBS.CS.CMU.EDU>]

 As part of a long-term project (MECHEM), we now have a strong need for
 some X Windows software that will draw molecular structures
 intelligibly from just a description of the connectivity.  That is,
 the software will have to determine planar coordinates for each atom,
 but just for viewability, not necessarily for verisimilitude.
 MECHEM generates molecular structures as it searches for reaction
 mechanisms, and we need an ability to conveniently inspect these
 structures.  Some simple hacks have been adequate for small
 hydrocarbons in heterogeneous catalysis, but now we wish to consider
 more complex molecules.
 I have searched the archives of this list, but didn't find what we
 need.  I would appreciate any advice.  Thank you.
 --
 Dr. Raul Valdes-Perez		Research Computer Scientist
 School of Computer Science	email: valdes %! at !% cs.cmu.edu
 Carnegie Mellon University	phone: 412 268 7127
 Pittsburgh, PA 15213 - USA	fax:   412 621 5117