text: The first theoretical study of the water dimer was an ab initio calculation published in 1968 by Morokuma and Pedersen. Since then, the water dimer has been the focus of sustained interest by theoretical chemists concerned with hydrogen bonding—a search of the CAS database up to 2006 returns over 1100 related references (73 of them in 2005).  In addition to serving as a model for hydrogen bonding, (HO) is thought to play a significant role in many atmospheric processes, including chemical reactions, condensation, and solar energy absorption by the atmosphere.   In addition, a complete understanding of the water dimer is thought to play a key role in a more thorough understanding of hydrogen bonding in liquid and solid forms of water.