UHF Spin Information
Spin Information:
N(alpha): 2
N(beta): 1
<S²>: 0.750000 (ideal: 0.750000)
Fields
| Field | Description |
|---|---|
| N(alpha) | Number of α electrons |
| N(beta) | Number of β electrons |
| ⟨S²⟩ | Expectation value of spin-squared operator |
| ideal | Ideal value for pure spin state: S(S+1) |
Spin Contamination
For a pure spin state, ⟨S²⟩ = S(S+1) where S = (N_α - N_β)/2.
| Multiplicity | S | Ideal ⟨S²⟩ |
|---|---|---|
| Singlet (1) | 0 | 0.0 |
| Doublet (2) | 1/2 | 0.75 |
| Triplet (3) | 1 | 2.0 |
| Quartet (4) | 3/2 | 3.75 |
Significant deviation from the ideal value indicates spin contamination (the determinant is not a pure spin eigenfunction). OpenQuantum reports ⟨S²⟩ for every UHF calculation.
Notes
- RHF/ROHF always have ⟨S²⟩ = ideal value (spin-pure)
- UHF may show spin contamination, especially for stretched bonds or transition metals
- ROHF is spin-pure but has a more complex Fock matrix construction