Ideas of Quantum Chemistry P73 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 686 13. Intermolecular Interactions Fig. . Part-entity relationship. Two distinct ways of dividing the H2O 2 system into subsystems. Division a is traditional. The interacting objects are two isolated water molecules and the interaction energy is equal to about 5 kcal mol attraction . Division b is more subtle - a certain point in space is treated as being composed of two fictitious charges q 0 and q and one of the charges is ascribed to one and the other to the second molecule. In this way two new subsystems are defined each of them composed of a water molecule and the corresponding point charge. The value of q may be chosen in such a way as to produce the interaction energy of the new subsystems close to 0. Therefore this is a more natural choice of subsystems than the traditional one. The total interaction energy of the two water molecules is now absorbed within the interactions of the fictitious point charges with their water molecules. Each of the point charges takes over the interaction of its water molecule with the rest of the Universe. Hence I have permitted myself with the necessary licentia poetica to use the yin and yang symbols - the two basic elements of ancient Chinese philosophy. Right between the molecules . in the middle of the OO separation we place two point charges q 0 and q . we place nothing since the charges cancel each other please compare a similar trick on p. 492 . We therefore have just two water molecules. Now we start our game. We say that the charges are real one belongs to one of the molecules and the other to the second Fig. . The charge q could be anything but we want to use it for a very special goal to construct the two subsystems in a more natural way than just two water molecules. It is interesting that after the choice is made any of the subsystems has lower energy than that of isolated water since the molecules are oriented in such a way as to attract each other. This means that the value of q can be chosen .
