the free, isolated vim der Waals and hydrogen-bonded molecules. Owing to the present state of experimental knowledge these are mostly dimers, i. e. , entities formed by two strongly bound molecules, an atom and a molecule, or two atoms held together by the weak hydrogen-bonding, or the still weaker van der Waals forces. Weakly bound complexes formed of more than two strongly bound sub-units, i. e. , trimers, tetramers, etc. , are now coming within reachof experimental observation and several papers in this book deal with them. The study of van der Waals and hydrogen-bonded interactions has been pursued for several decades. Most of these ,investigations have, however, dealt with systems in the condensed phase in which bulk effects are commingled with and therefore mask the weak binary interactions.