  Restricted open-shell Hartree-Fock (ROHF) theory is formulated as a projected
self-consistent unrestricted HF (UHF) model by mathematically constraining spin
density eigenvalues. The resulting constrained UHF (CUHF) wave function is
identical to that obtained from Roothaan's effective Fock operator. Our
$\alpha$ and $\beta$ CUHF Fock operators are parameter-free and have canonical
orbitals and orbital energies that are physically meaningful as in UHF, except
for eliminating spin contamination. The present approach removes ambiguities in
ROHF orbital energies and the non-uniqueness of methods that build upon them.
We present benchmarks to demonstrate CUHF physical correctness and good
agreement with experimental results.
