Taming the low-lying electronic states of FeH

The low-lying electronic states (X-4 Delta, A(4)Pi, a(6)Delta, b(6)Pi) 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 (T-0) and spectroscopic constants (r(e), r(0), B-e, B-0, (D) over bar (e), omega(e), v(0), alpha(e), omega(e)x(e)) were extrapolated to the valence complete basis set Douglas-Kroll (DK) aug-cc-pwCV infinity Z-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. The purely ab initio FPA approach yields the following T-0 results (in eV) for the lowest spin-orbit components of each electronic state: 0 (X-4 Delta) < 0.132 (A(4)Pi) < 0.190 (a(6)Delta) < 0.444 (b(6)Pi). The computed anharmonic fundamental vibrational frequencies (v(0)) for the (4,6)Delta electronic states are within 3 cm(-1) of experiment and provide reliable predictions for the (4,6)Pi 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. The FPA computations provide D-0 = 1.86 eV (42.9 kcal mol(-1)) for the 0 K dissociation energy of FeH and Delta H-f(298)degrees [FeH(g)] = 107.7 kcal mol(-1) for the enthalpy of formation at room temperature. 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 Delta -> A(4)Pi and a(6)Delta -> b(6)Pi transition energies are reproduced to 0.012 and 0.002 eV, respectively, while the error for the problematic X-4 Delta -> a(6)Delta intercombination excitation is reduced from at least 0.17 eV to about 0.04 eV. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767771]