Calculation of Extended Hartree-Fock Wavefunctions

It is shown that an extended Hartree‐Fock (EHF) function, obtained by applying a projection operator $\mathcal{O}$ to a Hartree product φ with subsequent optimization, has vanishing matrix elements of the operator $\mathfrak{H}\mathrm{‐EI}$ with all its singly excited states. A method for obtaining the EHF function using this property is described. Calculations for the Li atom are reported.