Seventy years ago Husimi introduced a new concept in the theory of N-particle systems: his Reduced Density Matrix (RDM) enabled one to discuss the properties of the whole system in terms of probability densities referring to only n particles at a time.
For n=1, the RDM gives the "particle density" (e.g. an electron density), while for n=2 it describes the "correlation" between the motions of two particles.
In the years that followed, the RDM has become evermore important. For electronic systems in particular, it is now possible to calculate the 2-electron RDM with an accuracy rivalling that of conventional (wave function) methods.
This short review touches not only the development of the methodology, but also its value in discussions of "separability", a topic that goes to the roots of quantum mechanics and continuing arguments of a philosophical nature. |