  A large collaboration carefully benchmarks 20 first principles many-body
electronic structure methods on a test set of 7 transition metal atoms, and
their ions and monoxides. Good agreement is attained between the 3
systematically converged methods, resulting in experiment-free reference
values. These reference values are used to assess the accuracy of modern
emerging and scalable approaches to the many-electron problem. The most
accurate methods obtain energies indistinguishable from experimental results,
with the agreement mainly limited by the experimental uncertainties. Comparison
between methods enables a unique perspective on calculations of many-body
systems of electrons.
