Quantum many-body dynamics representation

A molecule contains a nuclear distribution and an electronic distribution there is nothing else in a molecule. The nuclear arrangement is fully reflected in the electronic density distribution, consequently, the electronic density and its changes are sufficient to derive all information on all molecular properties. Molecular bodies are the fuzzy bodies of electronic charge density distributions consequently, the shape and shape changes of these fuzzy bodies potentially describe all molecular properties. Modern computational methods of quantum chemistry provide practical means to describe molecular electron distributions, and sufficiently accurate quantum chemical representations of the fuzzy molecular bodies are of importance for many reasons. A detailed analysis and understanding of "static" molecular properties such as "equilibrium" structure, and the more important dynamic properties such as vibrations, conformational changes and chemical reactions are hardly possible without a description of the molecule itself that implies a description of molecular bodies. 

How the correspondence principle should be applied to an atomic system thus depends critically on whether or not there exists a multiperiodic representation of the classical trajectories - the question first raised by Einstein. If the system possesses multiperiodic orbits, then its motion becomes separable, i.e. it becomes equivalent to as many independent modes as there are degrees of freedom. Dynamical separability is assumed in all independent particle and perturbative models of the many-electron atom. It is, however, not strictly applicable and the successes of simple quantum theory for many-electron systems are, to say the least, surprising. It was pointed out by Einstein, who based his arguments on the work of Poincare [519], that there exists no true separation of the three-body problem. 

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