Calculation of Metallocene Ionization Potentials via Auxiliary FieldQuantum Monte Carlo: Toward Benchmark Quantum Chemistry forTransition Metals

The accurateab initioprediction of ionization energies isessential to understanding the electrochemistry of transition metal complexesin both materials science and biological applications. However, suchpredictions have been complicated by the scarcity of gas phase experimentaldata, the relatively large size of the relevant molecules, and the presence ofstrong electron correlation effects. In this work, we apply all-electronphaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) utilizingmultideterminant trial wave functions to six metallocene complexes tocompare the computed adiabatic and vertical ionization energies withexperimental results. Wefind that ph-AFQMC yields mean absolute errors(MAEs) of 1.69 +/- 1.02 kcal/mol for the adiabatic energies and 2.85 +/- 1.13kcal/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 onevariant of localized coupled cluster calculations (DLPNO-CCSD(T0) with moderate PNO cutoffs), which has MAEs of 4.96 and6.08 kcal/mol, respectively. We also test the reliability of DLPNO-CCSD(T0) and DFT on acetylacetonate (acac) complexes foradiabatic energies measured in the same manner experimentally, and wefind 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 phaseresults.