other electronic state is found at 1.7 Å, which is a
5 A 2 state that results from a
combination of an s orbital and a d z 2 orbital localized on iron with A 1 symmetry, a
B 2 orbital, and a B 1 orbital, both of which are d orbitals. The
5 A 2 excited state is
83 kcal mol
−1 higher than the ground state. Beyond 2.0 Å, the calculations that
utilize the Hückel guess do not converge.
PBE0 does not perform much better (Fig. 17) than B3LYP. Around the minimum of the curve, the H core guess produces a smooth curve corresponding to a
5 A 2
state. This state is a combination of a B 1 orbital, a B 2 orbital, and two A 1 orbitals,
all of which are d orbitals. At 2.1 Å the calculation optimizes to an excited
5 A 1
state, which results from a combination of two d orbitals localized on iron with B 1
and B 2 symmetry, and the 2p x and 2p y orbitals, with B 1 and B 2 symmetry
Fig. 16 Points calculated for FeO at the B3LYP level of theory, using the H core and Hückel initial
guesses. The calculations do not converge past 2.0 Å using the Hückel guess or 2.2 Å using the
H core guess
Fig. 17 Points calculated for FeO at the PBE0 level of theory, using the H core and Hückel initial
guesses. The calculations do not converge past 2.0 Å using the Hückel guess or 2.1 Å using the
H core guess
The Importance of Orbital Analysis
23