Complex Absorbing Potential Equation-of-Motion Coupled-Cluster Method Yields Smooth and Internally Consistent Potential Energy Surfaces and Lifetimes for Molecular Resonances

The recently developed equation-of-motion electron-attachment coupled-cluster singles and doubles (EOM-EA-CCSD) method augmented by a complex absorbing potential (CAP) is applied to the (2)Pi(g) resonance of N-2(-) and the 2 Sigma(+)(u) resonance of H-2(-) at various intemudear distances. The results illustrate the advantages of EOM-CC for treating resonance states over state-specific approaches. CAP-EOM-EA-CCSD produces smoothly varying potential energy curves and lifetimes for both Sigma and Pi resonances. The computed lifetimes and energy differences between the neutral and electron-attached states are internally consistent, that is, the resonance width becomes zero at the same internuclear distance where the energy of the electron-attached state drops below that of the neutral state. Such smooth and internally consistent behavior is only achieved when the perturbation due to the CAP is removed using the first-order deperturbative correction that we introduced earlier; the evaluation of resonance positions and widths from raw (uncorrected) energies leads to unphysical discontinuities and fails to correctly describe the conversion of a resonance to a bound state at large intemudear distances.