Both Jim Cassatt (czj at cu.nih.gov) and J. Zsigo (zsigo at rs1.tcs.tulane.edu)
inquired about the program CHEMICAL. I believe the author is still in
business, as I have version 4.2 dated Feb. 23, 1990. There are several
other modules available in this package, but I haven't checked them out.
One of these is BIOCHEMICAL, which is supposed to model larger molecules.
I have also played around with a program called PCmodel, which is designed
for larger structures and can read PDB files for display and manipulation.
The registration fee for this one is more like $75 (Gee, and I've been meaning
to send that check in one of these days....!)
The most recent versions (or close to it) of both of these programs are
available as shareware from a SW house called The Software Labs. Their
number is 213-559-5456. The catalog # for CHEMICAL is 347, and for
PCmodel it's 349.
I have no connections with The Software Labs or the authors of either of
these two programs.