(3.3.16) Galli : We should push for a much more concerted effort of the different communities using DFT, aimed towards the verification and validation of properties calculated with DFAs. Different communities of DFT users, with different "cultures", still exist: the quantum-chemistry community, the solid-state and materials science community, and the community carrying out firstprinciples MD and interested in finite-temperature properties. It would be very important for these communities to come together and establish a list of properties, comparing results for molecular and solid-state systems . where the orbital occupations cannot be represented by a single Slater determinant. The classic example of a multireference system that cannot be represented by a single Slater determinant is the stretched H 2 molecule. The ground-state wave function in the dissociation limit is an equal mixture of two determinants. A simple single-determinant wave function from MO theory incorrectly includes both covalent and ionic terms and, consequently, gives in the dissociation limit an energy of only -11/16E h , compared to the exact energy of -1E h . Despite not using an explicit wave function, Kohn-Sham DFAs are valid only for single-determinant states and suffer from similar errors as Hartree-Fock-based wavefunction methods for multi-reference systems. This can be understood by examination of the pair density.