viii PREFACE road some years ago.This book was largely written and created at the Physical Chemistry Laboratory,Oxford,where both of us have spent a large part of our research careers.We owe a great debt of gratitude to the head of department,J.S.Rowlinson,who has provided us with continuous en- couragement and support in this venture,as well as a meticulous criticism of early versions of the manuscript.We would also like to thank our friends and colleagues in the Physics department at Bristol and the Chemistry depart- ment at Southampton for their help and encouragement,and we are indebted to many colleagues,who in discussions at conferences and workshops, particularly those organized by CCP5 and CECAM,have helped to form our ideas.We cannot mention all by name,but should say that conversations with D.Frenkel and P.A.Madden have been especially helpful.We would also like to thank M.Gillan and J.P.Ryckaert,who made useful comments on certain chapters,and I.R.McDonald who read and commented on the completed manuscript.We are grateful for the assistance of Mrs L.Hayes,at Oxford University Computing Service,where the original Microfiche was produced. Lastly,we thank Taylor and Francis for allowing us to reproduce diagrams from Molecular Physics and Advances in Physics,and ICL and Cray Research (UK)for the photographs in Fig.1.1.Detailed acknowledgements appear in the text. Books are not written without a lot of family support.One of us(DJT) wants to thank the Oaks and the Sibleys of Bicester for their hospitality during many weekends in the last three years.Our wives,Diane and Pauline,have suffered in silence during our frequent disappearances,and given us their unflagging support during the whole project.We owe them a great deal. Bristol M.P.A. Southampton D.J.T. May 1986