Abstract
A variational principle is found which implies that the Hartree‐Fock (HF) single‐particle Hamiltonian for occupied states is diagonal. The Brillouin condition is recovered by stipulating that the arbitrary unoccupied orbitals be orthogonal to the occupied ones. When it is further stipulated that the unoccupied orbitals be orthonormal, the HF equation for occupied states is determined uniquely and no off‐diagonal Lagrangian multipliers occur.

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