Copyright Statement. The publisher of J. Mol. Graphics, Butterworth-Heineman have asked the authors of this paper to clearly indicate the nature of the copyright associated with this article. They do not object to the paper being mounted in this form, but have asked that it be removed from the World-Wide-Webb once the article is published by Butterworth-Heinemann. They have asked that after this date, a statement to the effect that copyright would now had been transferred to the publisher as a result of publication of the paper in the journal, and that any further request for electrocopying and electrostorage should be referred to the publisher. The paper was published as Volume 12, page 226, and the contents of this electronic version are accordingly withdrawn. Readers should consult the printed version for further details.