Intermolecular forces in condensed phases

The origin of intermolecular forces in condensed phases is well known. However, the quantitative theoretical study of such intermolecular forces meets with great difficulties which are dependent on the fact that the distances between the molecules are of only the same order as the dimensions of the molecules themselves. The approximate formulae which are valid when the molecules are greater distances apart (say ten times the molecular diameter) as in a gas under normal conditions, lose all significance in a condensed phase, as we shall show later by an example. ... 

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