Calculation of Metallocene Ionization Potentials via Auxiliary Field Quantum Monte Carlo: Toward Benchmark Quantum Chemistry for Transition Metals

The
accurate ab initio prediction of ionization
energies is essential to understanding the electrochemistry of transition
metal complexes in both materials science and biological applications.
However, such predictions have been complicated by the scarcity of
gas phase experimental data, the relatively large size of the relevant
molecules, and the presence of strong electron correlation effects.
In this work, we apply all-electron phaseless auxiliary-field quantum
Monte Carlo (ph-AFQMC) utilizing multideterminant trial wave functions
to six metallocene complexes to compare the computed adiabatic and
vertical ionization energies with experimental results. We find that
ph-AFQMC yields mean absolute errors (MAEs) of 1.69 ± 1.02 kcal/mol
for the adiabatic energies and 2.85 ± 1.13 kcal/mol for the vertical
energies. We also carry out density functional theory (DFT) calculations
using a variety of functionals, which yields MAEs of 3.62–6.98
kcal/mol and 3.31–9.88 kcal/mol, as well as one variant of
localized coupled cluster calculations (DLPNO-CCSD­(T0)
with moderate PNO cutoffs), which has MAEs of 4.96 and 6.08 kcal/mol,
respectively. We also test the reliability of DLPNO-CCSD­(T0) and DFT on acetylacetonate (acac) complexes for adiabatic energies
measured in the same manner experimentally, and we find higher MAEs,
ranging from 4.56 to 10.99 kcal/mol (with a different ordering) for
DFT and 6.97 kcal/mol for DLPNO-CCSD­(T0). Finally, by utilizing
experimental solvation energies, we show that accurate reduction potentials
in solution for the metallocene series can be obtained from the AFQMC
gas phase results.