, where H N is the N-electron Hamiltonian, and Ψ app N and E app N are the computed approximations to the groundstate wave function and energy; this is a topic of ongoing research. In the DFT setting, a promising approach is the use of (non-guaranteed) estimates based on a statistical analysis of the model error -see, for example, ref. 341-343,346. Whether nonstatistical, guaranteed, accurate, and cheap-to-compute model error estimators can be constructed using DFT is a completely open question.