  Density functional theory is the workhorse of chemistry and materials
science, and novel density functional approximations (DFAs) are published every
year. To become available in program packages, the novel DFAs need to be
(re)implemented. However, according to our experience as developers of Libxc
[Lehtola et al, SoftwareX 7, 1 (2018)], a constant problem in this task is
verification, due to the lack of reliable reference data. As we discuss in this
work, this lack has lead to several non-equivalent implementations of
functionals such as BP86, PW91, PBE, and B3LYP across various program packages,
yielding different total energies. Through careful verification, we have also
found many issues with incorrect functional forms in recent DFAs.
  The goal of this work is to ensure the reproducibility of DFAs: DFAs must be
verifiable in order to prevent reappearances of the abovementioned errors and
incompatibilities. A common framework for verification and testing is therefore
needed. We suggest several ways in which reference energies can be produced
with free and open source software, either with non-self-consistent
calculations with tabulated atomic densities or via self-consistent
calculations with various program packages. The employed numerical parameters
-- especially, the quadrature grid -- need to be converged to guarantee a
$\lesssim0.1\mu E_{h}$ precision in the total energy, which is nowadays
routinely achievable in fully numerical calculations. Moreover, as such
sub-$\mu E_{h}$ level agreement can only be achieved when fully equivalent
implementations of the DFA are used, also the source code of the reference
implementation should be made available in any publication describing a new
DFA.
