International Workshop on 'New challenges in Reduced Density Matrix Functional Theory: Symmetries, time-evolution and entanglement'
Density matrix functionals: existence, uniqueness and applicabilityPeter Knowles
Cardiff University, Cardiff, United Kingdom
Coauthor(s) : Jian Wang
 Huzhou University, Zhejiang, China
Many popular approximations of the two-electron density matrix are formulated as algebraic functions of the one-electron density matrix, typically in the N-in-N active orbital space. Whilst good accuracy can often be achieved, there are examples of wavefunctions with identical one-electron, but strongly different two-electron density matrices, which mean that algebraic density-matrix functionals cannot be exact everywhere . The implications of this to the design of approximations to the static and dynamic correlation energy are discussed.
 J. Wang and P. J. Knowles, Phys. Rev. A 92 012520 (2015).