distance. Thus, any active space employed must account for both sources of multireference character. A sensible active space would then include the Fe 4s and
3d orbitals and the O 2p orbital set. While only two O 2p orbitals may take part in
bonding to Fe, the full set should initially be included due to the degeneracy of the
O 2p orbital set. Given this initial (12,9) active space, it may be possible to truncate
the active space if it can be shown that occupation of any orbitals remains constant
(doubly occupied or unoccupied) for all internuclear distances. It is known that the
ground state term of FeO is
5
D with A 2 symmetry, [21] while there also is a lowlying
5
Σ
+ state with A 1 symmetry known [39, 40]. If it is assumed that FeO exists as
Fig. 10 Points calculated for FeO at the ROHF level of theory, using the H core and Hückel initial
guesses and cc-pVTZ basis sets. The points at 2.4 and 3.1 Å on the H core curve and 2.5, 2.9, 3.2,
and 3.5 Å on the Hückel curve do not converge
Fig. 11 Points calculated for FeO at the UHF level of theory, using the H core and Hückel initial
guesses. The difference between the calculated 2
> and 2
> for the pure spin state
( 2
> calc − 2
> exact ) are inset
The Importance of Orbital Analysis
19