tailieunhanh - Ideas of Quantum Chemistry P77

Ideas of Quantum Chemistry P77 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. | 726 13. Intermolecular Interactions represent peanuts for the Padé They were already much better for L 3. Why are the Padé approximants so effective The apparent garbage produced by the perturbational series represented for the Padé approximants precise information that the absurd perturbational corrections pertain the energy of the 2pau state of the hydrogen atom in the electric field of the proton. How come Low-order perturbational corrections even if absolutely crazy somehow carry information about the physics of the problem. The convergence properties of the Rayleigh-Schrodinger perturbation theory depend critically on the poles of the function approximated see discussion on p. 210 . A pole cannot be described by any power series as happens in perturbation theories whereas the Padé approximants have poles built in the very essence of their construction the denominator as a polynomial . This is why they may fit so well with the nature of the problems under NON-ADDITIVITY OF INTERMOLECULAR INTERACTIONS Interaction energy represents the non-additivity of the total energy The total energy of interacting molecules is not an additive quantity . does not represent the sum of the energies of the isolated molecules. The reason for this non-additivity is the interaction energy. Let see whether the interaction energy itself has some additive properties. First of all the interaction energy requires the declaration of which fragments of the total system we treat as interacting molecules see beginning of this chapter . The only real system is the total system not these fragments. The fragments or subsystems can be chosen in many ways Fig. . If the theory is exact the total system can be described at any such choice cf. p. 492 . A theory has to be invariant with respect to any choice of subsystems in the system under consideration. Such a choice however in many cases apparently evident represents an arbitrary operation similar to the .

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