On the "killer condition'' in the equation-of-motion method: ionization potentials from multi-reference wave functions

The ionization operator Omega in the equation-of-motion (EOM) method is wri tten in a form that satisfies the "killer condition'' Omega (T)\Psi (0)] = 0 for arbitrary multiconfigurational reference states. The resulting equati on for ionization potentials is equivalent to traditional EOM equation only if the reference state is an exact eigenfunction of the Hamiltonian. The n ew equation is insensitive to specifying either a simple metric or the "com mutator metric'', and it represents a Hermitian formulation even for partia lly optimized wave functions. It is, however, equivalent to a multi-referen ce CI equation for the ionized state using the extended Koopmans ansatz.