Implications of unitary invariance for gradient theory

This paper shows how the invariance of the energy to unitary rotations in the occupied orbitals and in the virtual orbitals can be used to determine some elements of the derivative molecular orbitals, which are important for gradient theory. Solution of the usual coupled‐perturbed Hartree–Fock equations for these elements is thus unnecessary, eliminating considerable effort. We also give proofs which show clearly that special conditions are required.