Equation of motion coupled cluster method for electron attachment

Abstract

The electron attachment equation of motion coupled cluster (EA-EOMCC) method is derived which enables determination of the various bound states of an (N+1)-electron system and the corresponding energy eigenvalues relative to the energy of an N-electron CCSD reference state. Detailed working equations for the EA-EOMCC method are derived using diagrammatic techniques for both closed-shell and open-shell CCSD reference states based upon a single determinant. The EA-EOMCC method is applied to a variety of different problems, the main purpose being to establish its prospects and limitations. The results from EA-EOMCC calculations are compared to other EOMCC approaches, starting from different reference states, as well as other theoretical methods and experimental values, where available. We have investigated electron affinities for a wide selection of both closed-shell and open-shell systems. Excitation spectra of atoms and molecules with an odd number of electrons are obtained, taking the closed-shell ground state of the ion as a reference in the EA-EOMCC calculation. Finally we consider excitation spectra of some closed-shell systems, and find in particular that the electron attachment approach is capable of yielding accurate triplet excitation energies in an efficient way.