Angelo Vulpiani — Università di Roma La Sapienza #
A (random) tour around  reductionism and singular limits #
Reductionism, or more generally the relationship connecting the different sciences, is perhaps one of the few issues of the scientific culture still capable of stirring peremptory and robust discussions among dedicated scholars.
In spite of the famous Dirac' claim on  the reduction of chemistry to physics [{\it The fundamental laws necessary for the mathematical treatment of a large part of physics and the whole of chemistry are thus completely known, and the difficulty lies only in the fact that application of these laws leads to equations that are too complex to be solved}] even with the help of powerful computers and efficient numerical methods chemistry is not merely applied quantum mechanics.
A critical analysis  shows that classical mechanics is not just the $\hbar \to 0$ limiting case of quantum mechanics:for a chaotic classical system, the proper classical limit can be obtained from quantum mechanics only taking into account the influence of the outer environment.
It is correct to say that quantum mechanics plus (semi)classical physics is able to describe the observable features of molecules. The study of pyramidal molecules shows that the (semi)classical limit does not follow in a straightforward way from quantum mechanics but is a consequence of the interaction of single molecules with an external environment.