Investigations using the Becke-Roussel exchange functional

In 1989, Becke and Roussel introduced a new form for the exchange functional derived from the exchange-hole, and dependent on ϱ, | ∇ϱ |, ∇2ϱ and the kinetic energy density. Here we report calculations using this functional and show that functional derived from it compares well with those using the usual Becke88 exchange functiona. We also question whether it is correct to include atomisation energies in DFT optimisation databases, and we discuss alternatives such as energies of stretched molecules.