Manuscript received June 6;
1997;
revised October 3;
1997. This work was supported by NATO under Grant CRG 941284. The work of R. D. Coalson was supported in part by the NSF under Grant CHE-9529674. D. K. Pant and R. D. Coalson are with the Department of Chemistry;
University of Pittsburgh;
Pittsburgh;
PA 15260 USA. M. I. Hernández and J. Campos-Martínez are with the Instituto de Matemáticas y Física Fundamental (CSIC);
Serrano;
123;
E-28006;
Madrid;
Spain. Publisher Item Identifier S 0733-8724(98)00637-9;