IONIZATION-POTENTIALS AND ELECTRON-AFFINITIES FROM THE EXTENDED KOOPMANS THEOREM APPLIED TO ENERGY-DERIVATIVE DENSITY-MATRICES - THE EKTMPNAND EKTQCISD METHODS

The extended Koopmans' theorem (EKT) is combined with the energy-deriv ative formalism for the one-and two-particle reduced density matrices. Such a combination produces a versatile methodology for calculations of ionization potentials and electron affinities that, being applicabl e to any level of electron correlation treatment,is more general than the equation-of-motion (EOM) formalism. General expressions for the Fe ynman-Dyson amplitude, the pole strength, and the one-particle reduced density matrix of the hole state are derived. Like the electron propa gator theory (EPT), the present approach provides a one-electron descr iption of the electron attachment-detachment processes that is advanta geous from the interpretive point of view. Numerical tests show that E KT calculations are capable of affording ionization potentials with ac curacy comparable to that of the EPT methods but at a substantially lo wer computational cost. (C) 1997 American Institute of Physics. [S0021 -9606(97)03041-9].