George D. Purvis The chemical sample: A fundamental object for molecular modeling. 17-21 1994 34 Journal of Chemical Information and Computer Sciences 1 http://pubs.acs.org/cgi-bin/archive.cgi/jcisd8/1994/34/i01/f-pdf/f_ci00017a002.pdf db/journals/jcisd/jcisd34.html#Purvis94