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Abstract

The reaction FeO⁺ + H₂ → Fe⁺ + H₂O is a simple model for hydrogen abstraction processes in biologically important heme systems. The geometries of all relevant stationary points on the lowest sextet and quartet surfaces were optimized using several density functionals as well as the CASSCF method. The corresponding energy profiles were computed at the following levels: density
functional theory using gradient-corrected, hybrid, meta, hybrid-meta, and perturbatively corrected double hybrid
functionals; single-reference coupled cluster theory including up to single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]; correlated multireference ab initio methods (MRCI, MRAQCC, SORCI, SORCP, MRMP2, NEVPT2, and CASPT2). The calculated energies were corrected for scalar relativistic effects, zero-point vibrational energies, and core−valence correlation effects. MRCI and SORCI energies were corrected for size-consistency errors using an a posteriori Davidson correction (+Q) leading to MRCI+Q and SORCI+Q. Comparison with the available experimental data shows that CCSDT(Q) is most accurate and can thus serve as benchmark method for this electronically challenging reaction. Among the density functionals, B3LYP performs best. In the correlated ab initio calculations with a full-valence active space, SORCI+Q yields the lowest deviations from the CCSDT(Q) reference results, with qualitatively similar energy profiles being obtained from MRCI+Q and MRAQCC. SORCI+Q benefits from the quality of
the approximate average natural orbitals used in the final step of the SORCI procedure. Many of the tested methods show surprisingly large errors. The present results validate the common use of B3LYP in computational studies of heme systems and offer guidance on which correlated ab initio methods are most suitable for such studies.