Water dimer

[4] The ab initio binding energy between the two water molecules is estimated to be 5-6 kcal/mol, although values between 3 and 8 have been obtained depending on the method.

The experimentally measured dissociation energy (including nuclear quantum effects) of (H2O)2 and (D2O)2 are 3.16 ± 0.03 kcal/mol (13.22 ± 0.12 kJ/mol)[5] and 3.56 ± 0.03 kcal/mol (14.88 ± 0.12 kJ/mol),[6] respectively.

The vibrational ground-state is known as the linear water dimer (shown in the figure to the right), which is a near prolate top (viz., in terms of rotational constants[clarification needed], A > B ≈ C).

The first theoretical study of the water dimer was an ab initio calculation published in 1968 by Morokuma and Pedersen.

[10] 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).