The MOBH35 Metal–Organic Barrier Heights Reconsidered: Performance of Local-Orbital Coupled Cluster Approaches in Different Static Correlation Regimes

We have revisited
the MOBH35 (Metal–Organic Barrier Heights,
35 reactions) benchmark [Iron, Janes, J.
Phys. Chem. A, 2019, 123 (17), 3761−3781; ibid. 2019, 123, 6379–6380] for realistic organometallic catalytic reactions, using both canonical
CCSD­(T) and localized orbital approximations to it. For low levels
of static correlation, all of DLPNO-CCSD­(T), PNO-LCCSD­(T), and LNO-CCSD­(T)
perform well; for moderately strong levels of static correlation,
DLPNO-CCSD­(T) and (T1) may break down catastrophically,
and PNO-LCCSD­(T) is vulnerable as well. In contrast, LNO-CCSD­(T) converges
smoothly to the canonical CCSD­(T) answer with increasingly tight convergence
settings. The only two reactions for which our revised MOBH35 reference
values differ substantially from the original ones are reaction 9
and to a lesser extent 8, both involving iron. For the purpose of
evaluating density functional theory (DFT) methods for MOBH35, it
would be best to remove reaction 9 entirely as its severe level of
static correlation makes it just too demanding for a test. The magnitude
of the difference between DLPNO-CCSD­(T) and DLPNO-CCSD­(T1) is a reasonably good predictor for errors in DLPNO-CCSD­(T1) compared to canonical CCSD­(T); otherwise, monitoring all of T1, D1, max|tiA|, and 1/(εLUMO – εHOMO) should provide adequate warning
for potential problems. Our conclusions are not specific to the def2-SVP
basis set but are largely conserved for the larger def2-TZVPP, as
they are for the smaller def2-SV­(P): the latter may be an economical
choice for calibrating against canonical CCSD­(T). Finally, diagnostics
for static correlation are statistically clustered into groups corresponding
to (1) importance of single excitations in the wavefunction; (2a)
the small band gap, weakly separated from (2b) correlation entropy;
and (3) thermochemical importance of correlation energy, as well as
the slope of the DFT reaction energy with respect to the percentage
of HF exchange. Finally, a variable reduction analysis reveals that
much information on the multireference character is provided by T1, IND/Itot, and the exchange-based diagnostic A100[TPSS].