Hartree-Fock wavefunction

An antisymmetric single Slater determinant whose occupied orbitals make its energy stationary under occupied-virtual orbital rotations. A stationary solution need not be a stable or global minimum.

At Hartree-Fock stationarity, $⟨{\Phi }_0|H|{\Phi }_i^a⟩=0$ for each occupied-virtual single excitation.