  The bond dissociation energies of a set of 44 3d transition metal-containing
diatomics are computed with phaseless auxiliary-field quantum Monte Carlo
(ph-AFQMC) utilizing a correlated sampling technique. We investigate molecules
with H, N, O, F, Cl, and S ligands, including those in the 3dMLBE20 database
first compiled by Truhlar and co-workers with calculated and experimental
values that have since been revised by various groups. In order to make a
direct comparison of the accuracy of our ph-AFQMC calculations with previously
published results from 10 DFT functionals, CCSD(T), and icMR-CCSD(T), we
establish an objective selection protocol which utilizes the most recent
experimental results except for a few cases with well-specified discrepancies.
With the remaining set of 41 molecules, we find that ph-AFQMC gives robust
agreement with experiment superior to that of all other methods, with a mean
absolute error (MAE) of 1.4(4) kcal/mol and maximum error of 3(3) kcal/mol
(parenthesis account for reported experimental uncertainties and the
statistical errors of our ph-AFQMC calculations). In comparison, CCSD(T) and
B97, the best performing DFT functional considered here, have MAEs of 2.8 and
3.7 kcal/mol, respectively, and maximum errors in excess of 17 kcal/mol for
both methods. While a larger and more diverse data set would be required to
demonstrate that ph-AFQMC is truly a benchmark method for transition metal
systems, our results indicate that the method has tremendous potential,
exhibiting unprecedented consistency and accuracy compared to other approximate
quantum chemical approaches.
