Calculation of excitation energies of atomic systems using the operatorO^A^O^

An excitation Hamiltonian is formulated by adding an operator $\hat{O}\hat{A}\hat{O}$ to the Fock operation whose eigenvalue differences represent excitations of electronic systems. $\hat{O}$ is an operator which projects onto the virtual space of the Fock operator and $\hat{A}$ is chosen so that one has a Koopmans theorem and a variational principle for these states. The formalism is then used to calculate excitation energies of atomic He, Li, Be, and Na.