This dependence is also present for double excitations and is approximated in PCHB by contracting over all orbitals (see eq. 13). The additional dependence for single excitations is that the magnitude of the matrix element highly depends on the starting determinant, see eq. 6. For example, according to the Brillouin-Theorem, single excitations from the Hartree-Fock reference determinant (D HF ) should have a vanishing matrix element