- the small system size (today’s computers and DFT codes can routinely tackle periodic unit cells containing approximately 1,000 atoms or fewer)
- difficulty in modelling weak (that is, van der Waals) interactions
- dynamics over long time periods and properties that are not ground state (that is, finite temperatures or excited states).
- larger systems can be tackled by the linear scaling approach
- finite-temperature effects can be addressed through lattice dynamics and cluster expansion
- electronic excitations can be modelled using time-dependent DFT
- the GW method and the Bethe–Salpeter technique, and several approaches can model van der Waals interactions
improve the accuracy of calculations by developing more complex functionals.