Both initial guesses produce vastly different results at large internuclear distances. Near the equilibrium region, a
3 A 2 state is found which is a direct product of
B 1 and B 2 orbitals in the C 2v point group. These orbitals correspond to the π*
antibonding orbitals that result from the linear combination of 2p x atomic orbitals
and 2p y atomic orbitals, respectively. In the region of intermediate internuclear
distances (IID, the area of the potential energy curve between equilibrium and
dissociation) the points fall on a curve belonging to a state of
3 A 1 . This state results
from the direct product of two singly occupied B 1 orbitals corresponding to the π
and π* orbitals from the overlap of p x atomic orbitals. This gives an overall bond
order of 1, which is clearly on a different PEC than the ground state. As the
internuclear distance grows, the curve resulting from the Hückel initial guess begins
to oscillate drastically between different states. The upper state produced by the
Hückel guess leads to homolytic dissociation, while the lower state leads to a
heterolytic dissociation.
The triplet O 2 PEC also was calculated via UHF, as the unrestricted formalism
should be able to better describe the bond breaking region of the curve (see Fig. 2).
It is important to note that the ROHF wave function is an eigenvalue of the S
2
operator while the UHF wave function is not [38]. This means that UHF suffers
from spin contamination, as contributions from higher spin states are included in the
wave function. The
contamination within an unrestricted calculation. The
is S z (S z + 1). As increasingly higher spin states are included
The UHF curve calculated from Hückel guess produces one smooth, continuous
curve in the
3 A 1 state.
higher spin states, particularly in the IID region, the same state is found throughout
Fig. 2 Points calculated for O 2 at the UHF level of theory, using the H core and Hückel initial
guesses. The difference between the calculated
> and the
> for the pure spin state
(
> calc −
> exact ) are inset
The Importance of Orbital Analysis
11
3 A 2 state is found which is a direct product of
B 1 and B 2 orbitals in the C 2v point group. These orbitals correspond to the π*
antibonding orbitals that result from the linear combination of 2p x atomic orbitals
and 2p y atomic orbitals, respectively. In the region of intermediate internuclear
distances (IID, the area of the potential energy curve between equilibrium and
dissociation) the points fall on a curve belonging to a state of
3 A 1 . This state results
from the direct product of two singly occupied B 1 orbitals corresponding to the π
and π* orbitals from the overlap of p x atomic orbitals. This gives an overall bond
order of 1, which is clearly on a different PEC than the ground state. As the
internuclear distance grows, the curve resulting from the Hückel initial guess begins
to oscillate drastically between different states. The upper state produced by the
Hückel guess leads to homolytic dissociation, while the lower state leads to a
heterolytic dissociation.
The triplet O 2 PEC also was calculated via UHF, as the unrestricted formalism
should be able to better describe the bond breaking region of the curve (see Fig. 2).
It is important to note that the ROHF wave function is an eigenvalue of the S
2
operator while the UHF wave function is not [38]. This means that UHF suffers
from spin contamination, as contributions from higher spin states are included in the
wave function. The
contamination within an unrestricted calculation. The
is S z (S z + 1). As increasingly higher spin states are included
The UHF curve calculated from Hückel guess produces one smooth, continuous
curve in the
3 A 1 state.
higher spin states, particularly in the IID region, the same state is found throughout
Fig. 2 Points calculated for O 2 at the UHF level of theory, using the H core and Hückel initial
guesses. The difference between the calculated
> and the
> for the pure spin state
(
> calc −
> exact ) are inset
The Importance of Orbital Analysis
11