Hydrogen bonding and cohesive energy

In chapter i we discussed the interaction between two widely separated water molecules and concluded that it could conveniently be treated in terms of (i) the interaction between the permanent electric moments of the molecules, (ii) the polarization or delocalization terms due to the distortion of the electron cloud of one molecule by the field of the other, (iii) dispersion forces having their origin in co-ordinated electronic motion in the two molecules, and (iv) the repulsion due to the action of the Pauli exclusion principle when the two electron clouds begin to overlap. Most calculations of the cohesive energy of the ice crystal amount to an evaluation of these contributions for more or less realistic models. Among these it is convenient to distinguish two different types  

A hydrogen bond between two electronegative atoms is a structure in which the hydrogen lies roughly upon the line joining 

0-0 distance on each bond is small, about 2-76 A, and the hydrogen position is about one third of the way along each bond so that water molecules are preserved. The protons may not lie exactly upon each of the tetrahedrally directed bonds but are very close to doing so. 

Calculation of the energy of a hydrogen bond itself involves consideration of the contributions from each of the four effects noted above, and in addition some allowance must be made for the effects of the other intramolecular bonds formed by each of the oxygen atoms involved. In other words we must take some account of the fact that we are dealing with a bond between two water molecules with particular relative orientations and not with a simple 0-H... O structure. Despite these difficulties, however, it is possible to construct a first-order quantum-mechanical picture of the structure of the bond and from this to derive a reasonable estimate of its energy. 

The calculated energy of the bond between two molecules with 

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