ABSTRACT
Despite tremendous theoretical efforts, corroborated by state-of-the-art computational techniques toward the development of a genuine multireference coupled cluster (MRCC) theory, the search seems to be far from complete. This is due to the fact that a host of non-trivial problems are still present in the process of developing such approaches, and it now appears that it is really an arduous task to retain the simplistic beauty of a typical single-reference approach while formulating a multireference (MR) theory. The chief sources of difficulty in setting up MR theories are (a) preservation of size extensivity (connectedness) of the formalism, (b) the perfectly determined nature of the problem, that is, having an equal number of parameters and equations, (c) conservation of symmetry, (d) avoidance of intruder states, and (e) correlation to a solution of the projected Schrödinger equation.
