The equation-of-motion (EOM) coupled cluster (CC) method with full inclusion of the connected triple excitations for ionization energies has been formulated and implemented. Using proper factorization of the three- and four-body parts of the effective Hamiltonian, an efficient computational procedure has been proposed for IP-EOM-CCSDT which at the EOM level requires no-higher-than n(occ)(3)n(vir)(4) scaling. The method is calibrated by the evaluation of the valence vertical ionization potentials for CO, N-2, and F-2 molecules for several basis sets up to 160 basis functions. At the basis set limit, errors vary from 0.0 to 0.2 eV, compared to "experimental" vertical ionization potentials. (C) 2003 American Institute of Physics.