The low-lying electronic states (X 4?, A 4?, a 6?, b 6?) of the iron monohydride radical, which are especially troublesome for electronic structure theory, have been successfully described using a focal point analysis (FPA) approach that conjoined a correlation-consistent family of basis sets up to aug-cc-pwCV5Z-DK with high-order coupled cluster theory through hextuple (CCSDTQPH) excitations. Adiabatic excitation energies (T0) and spectroscopic constants (re, r0, Be, B0, e, we, v0, ae, wexe) were extrapolated to the valence complete basis set (CBS) Douglas-Kroll (DK) aug-cc-pwCV8Z-DK CCSDT level of theory, and additional treatments accounted for higher-order valence electron correlation, core correlation, spin-orbit coupling, and the diagonal Born-Oppenheimer correction (DBOC). The purely ab initio FPA approach yields the following T0 results (in eV) for the lowest spin-orbit components of each electronic state: 0.00 (X 4?) < 0.132 (A 4?) < 0.190 (a 6?) < 0.444 (b 6?). The computed anharmonic fundamental vibrational frequencies (v0) for the 4,6? electronic states are within 3 cm�1 of experiment and provide reliable predictions for the 4,6? states. With the cc-pVDZ basis set, even CCSDTQPH energies give an incorrect ground state of FeH, highlighting the importance of combining high-order electron correlation treatments with robust basis sets when studying transition-metal radicals. Despite sizable multireference character in the quartet states, high-order single-reference coupled cluster computations improve the spectroscopic parameters over previous multireference theoretical studies; for example, the X 4? -> A 4? and a 6? -> b 6? transition energies are reproduced to 0.28 and 0.05 kcal mol�1, respectively, while the error for the problematic X 4? -> a 6? intercombination excitation is reduced from at least 4 kcal mol�1 to about 1 kcal mol�1.

Taming the low-lying electronic states of FeH

245th National Meeting (2013)

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